diff --git a/apps/client/src/network/handlers/LobbyHandler.ts b/apps/client/src/network/handlers/LobbyHandler.ts
index 49ff35c..3b7e8a9 100644
--- a/apps/client/src/network/handlers/LobbyHandler.ts
+++ b/apps/client/src/network/handlers/LobbyHandler.ts
@@ -7,6 +7,8 @@
 import { contracts as protocol } from "@repo/shared";
 import type {
   LobbySettingsUpdatePayload,
+  SelectTeamPayload,
+  SelectTeamRejectedPayload,
   ServerToClientPayloadOf,
   StartGameRequestPayload,
 } from "@repo/shared";
@@ -31,6 +33,9 @@
   ) => void;
   startGame: (payload?: StartGameRequestPayload) => void;
   updateLobbySettings: (payload: LobbySettingsUpdatePayload) => void;
+  selectTeam: (payload: SelectTeamPayload) => void;
+  onSelectTeamRejected: (callback: (payload: SelectTeamRejectedPayload) => void) => void;
+  offSelectTeamRejected: (callback: (payload: SelectTeamRejectedPayload) => void) => void;
 };
 
 /** ロビー画面向けのソケットハンドラを生成する */
@@ -54,6 +59,15 @@
     updateLobbySettings: (payload) => {
       emitEvent(protocol.SocketEvents.LOBBY_SETTINGS_UPDATE, payload);
     },
+    selectTeam: (payload) => {
+      emitEvent(protocol.SocketEvents.SELECT_TEAM, payload);
+    },
+    onSelectTeamRejected: (callback) => {
+      onEvent(protocol.SocketEvents.SELECT_TEAM_REJECTED, callback);
+    },
+    offSelectTeamRejected: (callback) => {
+      offEvent(protocol.SocketEvents.SELECT_TEAM_REJECTED, callback);
+    },
   };
 };
 
diff --git a/apps/client/src/scenes/lobby/LobbyScene.tsx b/apps/client/src/scenes/lobby/LobbyScene.tsx
index ee9bdbc..f46abfe 100644
--- a/apps/client/src/scenes/lobby/LobbyScene.tsx
+++ b/apps/client/src/scenes/lobby/LobbyScene.tsx
@@ -1,6 +1,6 @@
 import { useEffect, useMemo, useState } from "react";
 import { domain } from "@repo/shared";
-import type { FieldSizePreset, StartGameRequestPayload } from "@repo/shared";
+import type { FieldSizePreset, StartGameRequestPayload, TeamAssignmentMode } from "@repo/shared";
 import { config } from "@client/config";
 import { socketManager } from "@client/network/SocketManager";
 import { OVERLAY_BUTTON_STYLE } from "@client/scenes/shared/styles/overlayStyles";
@@ -27,6 +27,18 @@
   LOBBY_WAITING_STYLE,
 } from "./styles/LobbyScene.styles";
 
+/** チームIDを1始まりの表示ラベルに変換する */
+const toTeamLabel = (teamId: number): string => `チーム${teamId + 1}`;
+
+/** チーム選択のオプション（チームID 0〜3 + ランダム） */
+const TEAM_SELECT_OPTIONS: Array<{ value: number | null; label: string }> = [
+  { value: null, label: "ランダム" },
+  { value: 0, label: toTeamLabel(0) },
+  { value: 1, label: toTeamLabel(1) },
+  { value: 2, label: toTeamLabel(2) },
+  { value: 3, label: toTeamLabel(3) },
+];
+
 type Props = {
   room: domain.room.Room | null;
   myId: string | null;
@@ -77,6 +89,9 @@
   }, []);
   const [selectedFieldSizePreset, setSelectedFieldSizePreset] =
     useState<FieldSizePreset>(config.GAME_CONFIG.DEFAULT_FIELD_PRESET);
+  const [selectedTeamAssignmentMode, setSelectedTeamAssignmentMode] =
+    useState<TeamAssignmentMode>("random");
+  const [teamFullMessage, setTeamFullMessage] = useState<string | null>(null);
   const [isRuleModalOpen, setIsRuleModalOpen] = useState(false);
   const [isStartConfirmVisible, setIsStartConfirmVisible] = useState(false);
   const [isSettingsModalOpen, setIsSettingsModalOpen] = useState(false);
@@ -104,8 +119,9 @@
     socketManager.lobby.updateLobbySettings({
       targetPlayerCount: selectedStartPlayerCount,
       fieldSizePreset: selectedFieldSizePreset,
+      teamAssignmentMode: selectedTeamAssignmentMode,
     });
-  }, [isMeOwner, selectedStartPlayerCount, selectedFieldSizePreset]);
+  }, [isMeOwner, selectedStartPlayerCount, selectedFieldSizePreset, selectedTeamAssignmentMode]);
 
   const handleStartClick = () => {
     setIsStartConfirmVisible(true);
@@ -123,6 +139,19 @@
     setIsStartConfirmVisible(false);
   };
 
+  useEffect(() => {
+    const handler = () => {
+      setTeamFullMessage("このチームは満員です。別のチームを選んでください。");
+      setTimeout(() => { setTeamFullMessage(null); }, 3000);
+    };
+    socketManager.lobby.onSelectTeamRejected(handler);
+    return () => { socketManager.lobby.offSelectTeamRejected(handler); };
+  }, []);
+
+  const handleSelectTeam = (preferredTeamId: number | null) => {
+    socketManager.lobby.selectTeam({ preferredTeamId });
+  };
+
   const toFieldPresetLabel = (preset: FieldSizePreset): string => {
     const range = config.GAME_CONFIG.FIELD_PRESETS[preset].recommendedPlayers;
     const baseLabel =
@@ -137,6 +166,13 @@
     return `${baseLabel} (${range.min}-${range.max}人目安)`;
   };
 
+  // 自分のチーム選択状態を取得する
+  const myPreferredTeamId = room.players.find((p) => p.id === myId)?.preferredTeamId ?? null;
+
+  // チーム割り当て方式のラベルを取得する
+  const teamAssignmentModeLabel =
+    room.teamAssignmentMode === "player_select" ? "プレイヤーが選択" : "ランダム";
+
   return (
     <>
       <style>{`
@@ -226,6 +262,12 @@
                         {toFieldPresetLabel(room.fieldSizePreset)}
                       </div>
                     </div>
+                    <div>
+                      <div style={LOBBY_HOST_SETTINGS_LABEL_STYLE}>チームの決め方</div>
+                      <div style={LOBBY_HOST_SETTINGS_VALUE_STYLE}>
+                        {teamAssignmentModeLabel}
+                      </div>
+                    </div>
                   </div>
 
                   <button
@@ -236,6 +278,36 @@
                   </button>
                 </div>
               )}
+
+              {/* player_selectモード時の全員向けチーム選択UI */}
+              {room.teamAssignmentMode === "player_select" && (
+                <div style={{ marginTop: 16 }}>
+                  <div style={LOBBY_HOST_SETTINGS_LABEL_STYLE}>チームを選ぶ</div>
+                  <div style={{ display: "flex", gap: 8, flexWrap: "wrap", marginTop: 8 }}>
+                    {TEAM_SELECT_OPTIONS.map(({ value, label }) => {
+                      const isSelected = myPreferredTeamId === value;
+                      return (
+                        <button
+                          key={String(value)}
+                          onClick={() => { handleSelectTeam(value); }}
+                          style={{
+                            ...OVERLAY_BUTTON_STYLE,
+                            opacity: isSelected ? 1 : 0.6,
+                            outline: isSelected ? "2px solid white" : "none",
+                          }}
+                        >
+                          {label}
+                        </button>
+                      );
+                    })}
+                  </div>
+                  {teamFullMessage && (
+                    <div style={{ marginTop: 8, color: "#ff6b6b", fontSize: "0.9rem", fontWeight: 700 }}>
+                      {teamFullMessage}
+                    </div>
+                  )}
+                </div>
+              )}
             </div>
           </div>
 
@@ -252,6 +324,11 @@
                     {p.name}
                   </span>
                   {p.isOwner && <span style={{ fontSize: "0.9em" }}>👑</span>}
+                  {room.teamAssignmentMode === "player_select" && (
+                    <span style={{ marginLeft: "auto", fontSize: "0.85em", opacity: 0.8 }}>
+                      {p.preferredTeamId !== null ? toTeamLabel(p.preferredTeamId) : "ランダム"}
+                    </span>
+                  )}
                   {p.isReady && (
                     <span style={{ marginLeft: "auto", fontSize: "0.9em" }}>
                       ✅
@@ -286,6 +363,8 @@
           selectedFieldSizePreset={selectedFieldSizePreset}
           onChangeFieldSizePreset={setSelectedFieldSizePreset}
           toFieldPresetLabel={toFieldPresetLabel}
+          selectedTeamAssignmentMode={selectedTeamAssignmentMode}
+          onChangeTeamAssignmentMode={setSelectedTeamAssignmentMode}
           onClose={() => { setIsSettingsModalOpen(false); }}
         />
       )}
diff --git a/apps/client/src/scenes/lobby/components/LobbySettingsModal.tsx b/apps/client/src/scenes/lobby/components/LobbySettingsModal.tsx
index 829ce14..8e63589 100644
--- a/apps/client/src/scenes/lobby/components/LobbySettingsModal.tsx
+++ b/apps/client/src/scenes/lobby/components/LobbySettingsModal.tsx
@@ -1,8 +1,9 @@
 /**
  * LobbySettingsModal
- * ホスト用のゲーム設定（人数・フィールドサイズ）を変更するモーダル
+ * ホスト用のゲーム設定（人数・フィールドサイズ・チーム割り当て方式）を変更するモーダル
  */
 import type { FieldSizePreset } from "@repo/shared";
+import type { TeamAssignmentMode } from "@repo/shared";
 import { OVERLAY_BUTTON_STYLE } from "@client/scenes/shared/styles/overlayStyles";
 import {
   LOBBY_SETTINGS_MODAL_BODY_STYLE,
@@ -23,6 +24,8 @@
   selectedFieldSizePreset: FieldSizePreset;
   onChangeFieldSizePreset: (preset: FieldSizePreset) => void;
   toFieldPresetLabel: (preset: FieldSizePreset) => string;
+  selectedTeamAssignmentMode: TeamAssignmentMode;
+  onChangeTeamAssignmentMode: (mode: TeamAssignmentMode) => void;
   onClose: () => void;
 };
 
@@ -35,6 +38,8 @@
   selectedFieldSizePreset,
   onChangeFieldSizePreset,
   toFieldPresetLabel,
+  selectedTeamAssignmentMode,
+  onChangeTeamAssignmentMode,
   onClose,
 }: LobbySettingsModalProps) => {
   return (
@@ -88,6 +93,26 @@
               ))}
             </select>
           </div>
+
+          <div style={LOBBY_SETTINGS_MODAL_FIELD_STYLE}>
+            <label
+              htmlFor="settings-modal-team-assignment-mode"
+              style={LOBBY_SETTINGS_MODAL_LABEL_STYLE}
+            >
+              チームの決め方
+            </label>
+            <select
+              id="settings-modal-team-assignment-mode"
+              value={selectedTeamAssignmentMode}
+              onChange={(event) => {
+                onChangeTeamAssignmentMode(event.target.value as TeamAssignmentMode);
+              }}
+              style={LOBBY_SETTINGS_MODAL_SELECT_STYLE}
+            >
+              <option value="random">ランダム（自動割り当て）</option>
+              <option value="player_select">プレイヤーが選択</option>
+            </select>
+          </div>
         </div>
 
         <div style={LOBBY_SETTINGS_MODAL_FOOTER_STYLE}>
diff --git a/apps/server/src/application/coordinators/startGameCoordinator.ts b/apps/server/src/application/coordinators/startGameCoordinator.ts
index f974987..aa608ed 100644
--- a/apps/server/src/application/coordinators/startGameCoordinator.ts
+++ b/apps/server/src/application/coordinators/startGameCoordinator.ts
@@ -113,6 +113,13 @@
     return;
   }
 
+  // player_selectモード時は各プレイヤーの希望チームIDを収集する
+  const teamPreferences = updatedRoom.teamAssignmentMode === "player_select"
+    ? Object.fromEntries(
+        updatedRoom.players.map((p) => [p.id, p.preferredTeamId]),
+      )
+    : undefined;
+
   startGameUseCase({
     roomId: updatedRoom.roomId,
     fieldConfig: {
@@ -122,6 +129,7 @@
     },
     playerIds: sessionPlayerIds,
     playerNamesById,
+    teamPreferences,
     gameSession: gameManager,
     bombStore: gameManager,
     onGameEnd: () => {
diff --git a/apps/server/src/domains/game/GameManager.ts b/apps/server/src/domains/game/GameManager.ts
index dc3ae97..6b01cd9 100644
--- a/apps/server/src/domains/game/GameManager.ts
+++ b/apps/server/src/domains/game/GameManager.ts
@@ -2,10 +2,7 @@
  * GameManager
  * ゲームセッション集合の生成，更新，参照管理を統括する
  */
-import type {
-  domain,
-  GameResultPayload,
-} from "@repo/shared";
+
 import { Player } from "./entities/player/Player.js";
 import { GameRoomSession } from "./application/services/GameRoomSession";
 import type { GameSessionCallbacks } from "./application/services/GameRoomSession";
@@ -75,12 +72,14 @@
     playerNamesById: Record<string, string>,
     fieldConfig: GameFieldConfig,
     callbacks: GameSessionCallbacks,
+    teamPreferences?: Record<string, number | null>,
   ) {
     this.lifecycleService.startRoomSession(
       playerIds,
       playerNamesById,
       fieldConfig,
       callbacks,
+      teamPreferences,
     );
   }
 
diff --git a/apps/server/src/domains/game/application/ports/gameUseCasePorts.ts b/apps/server/src/domains/game/application/ports/gameUseCasePorts.ts
index e053f6f..7c9f328 100644
--- a/apps/server/src/domains/game/application/ports/gameUseCasePorts.ts
+++ b/apps/server/src/domains/game/application/ports/gameUseCasePorts.ts
@@ -36,6 +36,7 @@
     playerNamesById: Record<string, string>,
     fieldConfig: GameFieldConfig,
     callbacks: GameSessionCallbacks,
+    teamPreferences?: Record<string, number | null>,
   ): void;
   getRoomStartTime(): number | undefined;
   getRoomFieldConfig(): GameFieldConfig | undefined;
diff --git a/apps/server/src/domains/game/application/services/GameRoomSession.ts b/apps/server/src/domains/game/application/services/GameRoomSession.ts
index a6ae809..e946f48 100644
--- a/apps/server/src/domains/game/application/services/GameRoomSession.ts
+++ b/apps/server/src/domains/game/application/services/GameRoomSession.ts
@@ -51,6 +51,7 @@
     playerIds: string[],
     playerNamesById: Record<string, string>,
     fieldConfig: GameFieldConfig,
+    teamPreferences?: Record<string, number | null>,
   ) {
     this.players = new Map();
     this.mapStore = new MapStore({
@@ -61,10 +62,11 @@
     this.fieldConfig = fieldConfig;
 
     playerIds.forEach((playerId) => {
-      // 現在のプレイヤー構成から人数が最も少ないチームを算出する
-      const assignedTeamId = TeamAssignmentService.getBalancedTeamId(
-        this.players,
-      );
+      // player_selectモードの希望チームIDがあればそれを使い，なければバランス割り当てする
+      const preferredTeamId = teamPreferences?.[playerId] ?? null;
+      const assignedTeamId = preferredTeamId !== null
+        ? preferredTeamId
+        : TeamAssignmentService.getBalancedTeamId(this.players);
 
       // 算出したチームIDを指定してプレイヤーを生成する
       const playerName = playerNamesById[playerId] ?? playerId;
diff --git a/apps/server/src/domains/game/application/services/GameSessionLifecycleService.ts b/apps/server/src/domains/game/application/services/GameSessionLifecycleService.ts
index 7536e13..b220b16 100644
--- a/apps/server/src/domains/game/application/services/GameSessionLifecycleService.ts
+++ b/apps/server/src/domains/game/application/services/GameSessionLifecycleService.ts
@@ -8,10 +8,6 @@
   ActiveBombRegistration,
   GameFieldConfig,
 } from "../ports/gameUseCasePorts";
-import type {
-  domain,
-  GameResultPayload,
-} from "@repo/shared";
 import { logEvent } from "@server/logging/logger";
 import {
   gameDomainLogEvents,
@@ -94,6 +90,7 @@
     playerNamesById: Record<string, string>,
     fieldConfig: GameFieldConfig,
     callbacks: GameSessionCallbacks,
+    teamPreferences?: Record<string, number | null>,
   ) {
     if (this.sessionRef.current) {
       logEvent(logScopes.GAME_SESSION_LIFECYCLE_SERVICE, {
@@ -110,6 +107,7 @@
       playerIds,
       playerNamesById,
       fieldConfig,
+      teamPreferences,
     );
 
     this.activePlayerIds.clear();
diff --git a/apps/server/src/domains/game/application/useCases/startGameUseCase.ts b/apps/server/src/domains/game/application/useCases/startGameUseCase.ts
index f3a2788..97a9589 100644
--- a/apps/server/src/domains/game/application/useCases/startGameUseCase.ts
+++ b/apps/server/src/domains/game/application/useCases/startGameUseCase.ts
@@ -21,6 +21,7 @@
   fieldConfig: GameFieldConfig;
   playerIds: string[];
   playerNamesById: Record<string, string>;
+  teamPreferences?: Record<string, number | null>;
   gameSession: StartGamePort;
   bombStore: BombPlacementPort;
   onGameEnd: () => void;
@@ -109,6 +110,7 @@
   fieldConfig,
   playerIds,
   playerNamesById,
+  teamPreferences,
   gameSession,
   bombStore,
   onGameEnd,
@@ -156,6 +158,7 @@
         });
       },
     },
+    teamPreferences,
   );
 
   const startTime = gameSession.getRoomStartTime() || Date.now();
diff --git a/apps/server/src/domains/room/RoomManager.ts b/apps/server/src/domains/room/RoomManager.ts
index 8c38f8c..10ec32f 100644
--- a/apps/server/src/domains/room/RoomManager.ts
+++ b/apps/server/src/domains/room/RoomManager.ts
@@ -11,6 +11,7 @@
   JoinRoomResult,
   RoomDisconnectResult,
   RoomPhaseTransitionResult,
+  SelectTeamResult,
 } from "./application/ports/roomUseCasePorts";
 
 /** ルーム操作の公開インターフェースを提供するマネージャ */
@@ -63,11 +64,12 @@
     return this.roomPhaseService.markRoomWaiting(roomId);
   }
 
-  // ロビー設定（ゲーム人数・フィールドサイズ）を更新してルームを返す
+  // ロビー設定（ゲーム人数・フィールドサイズ・チーム割り当て方式）を更新してルームを返す
   public updateLobbySettings(
     roomId: string,
     targetPlayerCount: number,
     fieldSizePreset: domain.room.Room["fieldSizePreset"],
+    teamAssignmentMode: domain.room.TeamAssignmentMode,
   ): domain.room.Room | undefined {
     const room = this.rooms.get(roomId);
     if (!room || room.status !== domain.room.RoomPhase.WAITING) {
@@ -76,9 +78,43 @@
 
     room.targetPlayerCount = targetPlayerCount;
     room.fieldSizePreset = fieldSizePreset;
+    room.teamAssignmentMode = teamAssignmentMode;
     return room;
   }
 
+  // プレイヤーのチーム選択を更新して所属ルームを返す
+  // プレイヤーのチーム選択を更新する，チームが満員なら team_full を返す
+  public selectTeam(
+    playerId: string,
+    preferredTeamId: number | null,
+  ): SelectTeamResult {
+    const room = this.roomQueryService.getRoomByPlayerId(playerId);
+    if (!room || room.status !== domain.room.RoomPhase.WAITING) {
+      return { status: "not_found" };
+    }
+
+    const member = room.players.find((p) => p.id === playerId);
+    if (!member) {
+      return { status: "not_found" };
+    }
+
+    // チーム指定がある場合は人数上限を確認する（上限 = ゲーム人数 / 4）
+    if (preferredTeamId !== null) {
+      const maxPerTeam = Math.floor(
+        (room.targetPlayerCount ?? room.maxPlayers) / 4,
+      );
+      const currentCount = room.players.filter(
+        (p) => p.preferredTeamId === preferredTeamId && p.id !== playerId,
+      ).length;
+      if (currentCount >= maxPerTeam) {
+        return { status: "team_full", teamId: preferredTeamId };
+      }
+    }
+
+    member.preferredTeamId = preferredTeamId;
+    return { status: "ok", room };
+  }
+
   // ルームを削除する
   public deleteRoom(roomId: string): boolean {
     return this.rooms.delete(roomId);
diff --git a/apps/server/src/domains/room/application/ports/roomUseCasePorts.ts b/apps/server/src/domains/room/application/ports/roomUseCasePorts.ts
index f6c135d..4c73b59 100644
--- a/apps/server/src/domains/room/application/ports/roomUseCasePorts.ts
+++ b/apps/server/src/domains/room/application/ports/roomUseCasePorts.ts
@@ -35,6 +35,7 @@
 export interface RoomOutputPort {
   publishRoomUpdateToRoom(roomId: domain.room.Room["roomId"], room: domain.room.Room): void;
   publishJoinRejectedToSocket(payload: domain.room.JoinRoomRejectedPayload): void;
+  publishSelectTeamRejectedToSocket(teamId: number): void;
 }
 
 /** ルーム参加ユースケースが利用する参加操作ポート */
@@ -91,9 +92,24 @@
     roomId: string,
     targetPlayerCount: number,
     fieldSizePreset: domain.room.Room["fieldSizePreset"],
+    teamAssignmentMode: domain.room.Room["teamAssignmentMode"],
   ): domain.room.Room | undefined;
 }
 
+/** チーム選択の結果 */
+export type SelectTeamResult =
+  | { status: "ok"; room: domain.room.Room }
+  | { status: "team_full"; teamId: number }
+  | { status: "not_found" };
+
+/** チーム選択操作ポート */
+export interface SelectTeamPort {
+  selectTeam(
+    playerId: string,
+    preferredTeamId: domain.room.RoomMember["preferredTeamId"],
+  ): SelectTeamResult;
+}
+
 /** ルーム参加後にゲームランタイムを確保する操作ポート */
 export interface EnsureGameRuntimePort {
   ensureGameManagerForRoom(roomId: string): void;
diff --git a/apps/server/src/domains/room/application/services/RoomJoinService.ts b/apps/server/src/domains/room/application/services/RoomJoinService.ts
index 2dc89ab..7e6cf0f 100644
--- a/apps/server/src/domains/room/application/services/RoomJoinService.ts
+++ b/apps/server/src/domains/room/application/services/RoomJoinService.ts
@@ -22,6 +22,7 @@
         status: domain.room.RoomPhase.WAITING,
         maxPlayers: config.GAME_CONFIG.MAX_PLAYERS_PER_ROOM,
         fieldSizePreset: config.GAME_CONFIG.DEFAULT_FIELD_PRESET,
+        teamAssignmentMode: "random",
       };
       this.rooms.set(roomId, room);
       logEvent(logScopes.ROOM_JOIN_SERVICE, {
@@ -75,6 +76,7 @@
       name: playerName,
       isOwner: room.ownerId === socketId,
       isReady: false,
+      preferredTeamId: null,
     };
 
     room.players.push(newPlayer);
diff --git a/apps/server/src/logging/contracts/payloadByScope.ts b/apps/server/src/logging/contracts/payloadByScope.ts
index 05ffae0..6232381 100644
--- a/apps/server/src/logging/contracts/payloadByScope.ts
+++ b/apps/server/src/logging/contracts/payloadByScope.ts
@@ -75,6 +75,13 @@
   socketId: string;
 };
 
+/** NetworkのSELECT_TEAM不正ペイロードログ契約 */
+type NetworkSelectTeamLogPayload = {
+  event: typeof protocol.SocketEvents.SELECT_TEAM;
+  result: typeof logResults.IGNORED_INVALID_PAYLOAD;
+  socketId: string;
+};
+
 /** Networkスコープのログ契約ユニオン */
 type NetworkLogPayload =
   | NetworkConnectLogPayload
@@ -84,7 +91,8 @@
   | NetworkMoveLogPayload
   | NetworkPlaceBombLogPayload
   | NetworkBombHitReportLogPayload
-  | NetworkLobbySettingsUpdateLogPayload;
+  | NetworkLobbySettingsUpdateLogPayload
+  | NetworkSelectTeamLogPayload;
 
 /** GameUseCaseのSTART_GAMEログ契約 */
 type GameUseCaseStartGameLogPayload = {
diff --git a/apps/server/src/network/handlers/payloadGuard.ts b/apps/server/src/network/handlers/payloadGuard.ts
index ea3e8d5..e0927e2 100644
--- a/apps/server/src/network/handlers/payloadGuard.ts
+++ b/apps/server/src/network/handlers/payloadGuard.ts
@@ -35,6 +35,10 @@
     event: protocol.SocketEvents.LOBBY_SETTINGS_UPDATE,
     result: logResults.IGNORED_INVALID_PAYLOAD,
   },
+  [protocol.SocketEvents.SELECT_TEAM]: {
+    event: protocol.SocketEvents.SELECT_TEAM,
+    result: logResults.IGNORED_INVALID_PAYLOAD,
+  },
 } as const;
 
 type PayloadGuardEventName = keyof typeof invalidPayloadLogByEvent;
diff --git a/apps/server/src/network/handlers/room/createRoomOutputAdapter.ts b/apps/server/src/network/handlers/room/createRoomOutputAdapter.ts
index daf7957..e0e7801 100644
--- a/apps/server/src/network/handlers/room/createRoomOutputAdapter.ts
+++ b/apps/server/src/network/handlers/room/createRoomOutputAdapter.ts
@@ -28,6 +28,12 @@
     publishJoinRejectedToSocket: (payload: domain.room.JoinRoomRejectedPayload) => {
       reliable.emitToSocket(protocol.SocketEvents.ROOM_JOIN_REJECTED, payload);
     },
+    publishSelectTeamRejectedToSocket: (teamId: number) => {
+      reliable.emitToSocket(protocol.SocketEvents.SELECT_TEAM_REJECTED, {
+        preferredTeamId: teamId,
+        reason: "team_full" as const,
+      });
+    },
   };
 };
 
diff --git a/apps/server/src/network/handlers/room/registerRoomHandlers.ts b/apps/server/src/network/handlers/room/registerRoomHandlers.ts
index 92d2bde..1512acf 100644
--- a/apps/server/src/network/handlers/room/registerRoomHandlers.ts
+++ b/apps/server/src/network/handlers/room/registerRoomHandlers.ts
@@ -8,27 +8,34 @@
   JoinRoomEventRoomUseCasePort,
   JoinRoomEventRuntimeUseCasePort,
   LobbySettingsUpdateEventRoomUseCasePort,
+  SelectTeamEventRoomUseCasePort,
 } from "@server/network/types/connectionPorts";
 import { createSocketRegistrationContext } from "@server/network/handlers/registration";
 import {
   isJoinRoomPayload,
   isLobbySettingsUpdatePayload,
+  isSelectTeamPayload,
 } from "@server/network/validation/socketPayloadValidators";
 import type { RoomOutputAdapter } from "./createRoomOutputAdapter";
-import type { LobbySettingsUpdatePayload } from "@repo/shared";
+import type { LobbySettingsUpdatePayload, SelectTeamPayload } from "@repo/shared";
 import {
   handleJoinRoomEvent,
   handleLobbySettingsUpdateEvent,
+  handleSelectTeamEvent,
   type JoinRoomEventPayload,
   type JoinRoomOrchestratorDeps,
   type LobbySettingsUpdateOrchestratorDeps,
+  type SelectTeamOrchestratorDeps,
 } from "./roomEventOrchestrators";
 import {
   registerGuardedEvent,
   type GuardedEventDefinition,
 } from "@server/network/handlers/eventDefinitionRegistrar";
 
-type RoomHandlerRoomUseCasePort = JoinRoomEventRoomUseCasePort & LobbySettingsUpdateEventRoomUseCasePort;
+type RoomHandlerRoomUseCasePort =
+  & JoinRoomEventRoomUseCasePort
+  & LobbySettingsUpdateEventRoomUseCasePort
+  & SelectTeamEventRoomUseCasePort;
 
 type JoinRoomEventDefinition = GuardedEventDefinition<
   typeof protocol.SocketEvents.JOIN_ROOM,
@@ -40,6 +47,11 @@
   LobbySettingsUpdatePayload
 >;
 
+type SelectTeamEventDefinition = GuardedEventDefinition<
+  typeof protocol.SocketEvents.SELECT_TEAM,
+  SelectTeamPayload
+>;
+
 /** ルームイベント調停で利用する依存束を生成する */
 const createJoinRoomOrchestratorDeps = (
   socket: Socket,
@@ -71,6 +83,19 @@
   };
 };
 
+/** チーム選択イベント調停で利用する依存束を生成する */
+const createSelectTeamOrchestratorDeps = (
+  socket: Socket,
+  roomManager: SelectTeamEventRoomUseCasePort,
+  roomOutputAdapter: RoomOutputAdapter,
+): SelectTeamOrchestratorDeps => {
+  return {
+    socketId: socket.id,
+    roomManager,
+    output: roomOutputAdapter,
+  };
+};
+
 /** JOIN_ROOMイベント定義を生成する */
 const createJoinRoomEventDefinition = (
   deps: JoinRoomOrchestratorDeps,
@@ -97,6 +122,19 @@
   };
 };
 
+/** SELECT_TEAMイベント定義を生成する */
+const createSelectTeamEventDefinition = (
+  deps: SelectTeamOrchestratorDeps,
+): SelectTeamEventDefinition => {
+  return {
+    event: protocol.SocketEvents.SELECT_TEAM,
+    validator: isSelectTeamPayload,
+    orchestrate: (payload) => {
+      handleSelectTeamEvent(deps, payload);
+    },
+  };
+};
+
 /** ルーム関連イベントを検証してユースケースへ連携する */
 export const registerRoomHandlers = (
   socket: Socket,
@@ -117,7 +155,13 @@
     roomManager,
     roomOutputAdapter,
   );
+  const selectTeamDeps = createSelectTeamOrchestratorDeps(
+    socket,
+    roomManager,
+    roomOutputAdapter,
+  );
 
   registerGuardedEvent(onEvent, guardOnEvent, createJoinRoomEventDefinition(joinRoomDeps));
   registerGuardedEvent(onEvent, guardOnEvent, createLobbySettingsUpdateEventDefinition(lobbySettingsDeps));
+  registerGuardedEvent(onEvent, guardOnEvent, createSelectTeamEventDefinition(selectTeamDeps));
 };
diff --git a/apps/server/src/network/handlers/room/roomEventOrchestrators.ts b/apps/server/src/network/handlers/room/roomEventOrchestrators.ts
index c0eeb23..35b97c6 100644
--- a/apps/server/src/network/handlers/room/roomEventOrchestrators.ts
+++ b/apps/server/src/network/handlers/room/roomEventOrchestrators.ts
@@ -5,7 +5,7 @@
  * 本ファイルではランタイム未解決ログ対象イベントを扱わない
  */
 import { domain } from "@repo/shared";
-import type { LobbySettingsUpdatePayload } from "@repo/shared";
+import type { LobbySettingsUpdatePayload, SelectTeamPayload } from "@repo/shared";
 import { joinRoomUseCase } from "@server/domains/room/application/useCases/joinRoomUseCase";
 import { logEvent } from "@server/logging/logger";
 import { logResults, logScopes, roomUseCaseLogEvents } from "@server/logging/index";
@@ -13,6 +13,7 @@
   JoinRoomEventRoomUseCasePort,
   JoinRoomEventRuntimeUseCasePort,
   LobbySettingsUpdateEventRoomUseCasePort,
+  SelectTeamEventRoomUseCasePort,
 } from "@server/network/types/connectionPorts";
 import type { RoomOutputAdapter } from "./createRoomOutputAdapter";
 
@@ -35,6 +36,13 @@
   output: RoomOutputAdapter;
 };
 
+/** SELECT_TEAMイベント調停で利用する依存集合 */
+export type SelectTeamOrchestratorDeps = {
+  socketId: string;
+  roomManager: SelectTeamEventRoomUseCasePort;
+  output: RoomOutputAdapter;
+};
+
 /** LOBBY_SETTINGS_UPDATEイベントを調停してルーム設定を更新し全員に通知する */
 export const handleLobbySettingsUpdateEvent = (
   deps: LobbySettingsUpdateOrchestratorDeps,
@@ -49,6 +57,7 @@
     room.roomId,
     payload.targetPlayerCount,
     payload.fieldSizePreset,
+    payload.teamAssignmentMode,
   );
   if (!updatedRoom) {
     return;
@@ -57,6 +66,26 @@
   deps.output.publishRoomUpdateToRoom(room.roomId, updatedRoom);
 };
 
+/** SELECT_TEAMイベントを調停してプレイヤーのチーム選択を更新し全員に通知する */
+export const handleSelectTeamEvent = (
+  deps: SelectTeamOrchestratorDeps,
+  payload: SelectTeamPayload,
+): void => {
+  const result = deps.roomManager.selectTeam(
+    deps.socketId,
+    payload.preferredTeamId,
+  );
+
+  if (result.status === "team_full") {
+    deps.output.publishSelectTeamRejectedToSocket(result.teamId);
+    return;
+  }
+
+  if (result.status === "ok") {
+    deps.output.publishRoomUpdateToRoom(result.room.roomId, result.room);
+  }
+};
+
 /** JOIN_ROOMイベントを調停して参加ユースケースを実行する */
 export const handleJoinRoomEvent = async (
   deps: JoinRoomOrchestratorDeps,
diff --git a/apps/server/src/network/types/connectionPorts.ts b/apps/server/src/network/types/connectionPorts.ts
index 734602e..c3e0ab1 100644
--- a/apps/server/src/network/types/connectionPorts.ts
+++ b/apps/server/src/network/types/connectionPorts.ts
@@ -18,6 +18,7 @@
   FindRoomByPlayerPort,
   JoinRoomPort,
   RoomPhaseTransitionPort,
+  SelectTeamPort,
   UpdateLobbySettingsPort,
 } from "@server/domains/room/application/ports/roomUseCasePorts";
 
@@ -60,13 +61,19 @@
   "ensureGameManagerForRoom"
 >;
 
+/** チーム選択イベント調停で利用するルーム依存ポート */
+export type SelectTeamEventRoomUseCasePort =
+  & Pick<ConnectionRoomPort, "getRoomByPlayerId">
+  & SelectTeamPort;
+
 /** ソケット接続全体で利用するルーム管理ポート集合 */
 export type SocketConnectionRoomPort =
   & ConnectionRoomPort
   & DisconnectRoomPort
   & FindRoomByIdPort
   & DeleteRoomPort
-  & UpdateLobbySettingsPort;
+  & UpdateLobbySettingsPort
+  & SelectTeamPort;
 
 /** ソケット接続全体で利用するランタイム管理ポート集合 */
 export type SocketConnectionRuntimePort =
diff --git a/apps/server/src/network/validation/socketPayloadValidators.ts b/apps/server/src/network/validation/socketPayloadValidators.ts
index c03ecf2..8acb7f8 100644
--- a/apps/server/src/network/validation/socketPayloadValidators.ts
+++ b/apps/server/src/network/validation/socketPayloadValidators.ts
@@ -7,6 +7,7 @@
   LobbySettingsUpdatePayload,
   PlaceBombPayload,
   BombHitReportPayload,
+  SelectTeamPayload,
   StartGameRequestPayload,
 } from "@repo/shared";
 import type { PingPayload } from "@repo/shared";
@@ -112,7 +113,7 @@
   }
 
   const candidate = value as Record<string, unknown>;
-  const { targetPlayerCount, fieldSizePreset } = candidate;
+  const { targetPlayerCount, fieldSizePreset, teamAssignmentMode } = candidate;
 
   const isValidCount = (
     typeof targetPlayerCount === "number" &&
@@ -127,7 +128,29 @@
     || fieldSizePreset === "XLARGE"
   );
 
-  return isValidCount && isValidPreset;
+  const isValidMode = (
+    teamAssignmentMode === "random"
+    || teamAssignmentMode === "player_select"
+  );
+
+  return isValidCount && isValidPreset && isValidMode;
+};
+
+/** SELECT_TEAMイベントのペイロードがチーム選択情報であるか判定する */
+export const isSelectTeamPayload = (
+  value: unknown,
+): value is SelectTeamPayload => {
+  if (typeof value !== "object" || value === null) {
+    return false;
+  }
+
+  const candidate = value as Record<string, unknown>;
+  const { preferredTeamId } = candidate;
+
+  return (
+    preferredTeamId === null
+    || (typeof preferredTeamId === "number" && Number.isInteger(preferredTeamId) && preferredTeamId >= 0)
+  );
 };
 
 /** JOIN_ROOMイベントのペイロードが参加情報であるか判定する */
diff --git a/packages/shared/src/domains/room/index.ts b/packages/shared/src/domains/room/index.ts
index cd19cd4..99d37a6 100644
--- a/packages/shared/src/domains/room/index.ts
+++ b/packages/shared/src/domains/room/index.ts
@@ -15,4 +15,5 @@
   JoinRoomPayload,
   JoinRoomRejectedReason,
   JoinRoomRejectedPayload,
+  TeamAssignmentMode,
 } from "./room.type";
diff --git a/packages/shared/src/domains/room/room.type.ts b/packages/shared/src/domains/room/room.type.ts
index fa2af80..a9278b4 100644
--- a/packages/shared/src/domains/room/room.type.ts
+++ b/packages/shared/src/domains/room/room.type.ts
@@ -9,12 +9,17 @@
 /** ルーム進行フェーズ状態型 */
 export type RoomPhase = "waiting" | "playing" | "result";
 
+/** チーム割り当て方式 */
+export type TeamAssignmentMode = "random" | "player_select";
+
 /** ルーム所属プレイヤー情報 */
 export interface RoomMember {
   id: string;
   name: string;
   isOwner: boolean;
   isReady: boolean;
+  /** player_selectモード時にプレイヤーが選択したチームID（null = ランダム） */
+  preferredTeamId: number | null;
 }
 
 /** ルーム全体状態データ */
@@ -27,6 +32,8 @@
   fieldSizePreset: FieldSizePreset;
   /** ホストがロビーで選択したゲーム参加人数 */
   targetPlayerCount?: number;
+  /** ホストが選択したチーム割り当て方式 */
+  teamAssignmentMode: TeamAssignmentMode;
 }
 
 /** ルーム参加時に送信するペイロード */
diff --git a/packages/shared/src/protocol/eventPayloads.ts b/packages/shared/src/protocol/eventPayloads.ts
index 3fd765c..972f0f3 100644
--- a/packages/shared/src/protocol/eventPayloads.ts
+++ b/packages/shared/src/protocol/eventPayloads.ts
@@ -13,6 +13,9 @@
   LobbySettingsUpdatePayload,
   RoomJoinRejectedPayload,
   RoomUpdatePayload,
+  SelectTeamPayload,
+  SelectTeamRejectedPayload,
+  TeamAssignmentMode,
 } from "./payloads/lobbyPayloads";
 
 /** ゲームイベントのペイロード型を再公開する */
diff --git a/packages/shared/src/protocol/maps/lobbyEventPayloadMap.ts b/packages/shared/src/protocol/maps/lobbyEventPayloadMap.ts
index fcb3873..0df7613 100644
--- a/packages/shared/src/protocol/maps/lobbyEventPayloadMap.ts
+++ b/packages/shared/src/protocol/maps/lobbyEventPayloadMap.ts
@@ -9,16 +9,20 @@
   LobbySettingsUpdatePayload,
   RoomJoinRejectedPayload,
   RoomUpdatePayload,
+  SelectTeamPayload,
+  SelectTeamRejectedPayload,
 } from "../payloads/lobbyPayloads";
 
 /** ロビー関連のクライアント送信イベントペイロード対応表 */
 export type LobbyClientToServerEventPayloadMap = {
   [SocketEvents.JOIN_ROOM]: JoinRoomPayload;
   [SocketEvents.LOBBY_SETTINGS_UPDATE]: LobbySettingsUpdatePayload;
+  [SocketEvents.SELECT_TEAM]: SelectTeamPayload;
 };
 
 /** ロビー関連のサーバー送信イベントペイロード対応表 */
 export type LobbyServerToClientEventPayloadMap = {
   [SocketEvents.ROOM_JOIN_REJECTED]: RoomJoinRejectedPayload;
   [SocketEvents.ROOM_UPDATE]: RoomUpdatePayload;
+  [SocketEvents.SELECT_TEAM_REJECTED]: SelectTeamRejectedPayload;
 };
diff --git a/packages/shared/src/protocol/payloads/lobbyPayloads.ts b/packages/shared/src/protocol/payloads/lobbyPayloads.ts
index 171984e..518497d 100644
--- a/packages/shared/src/protocol/payloads/lobbyPayloads.ts
+++ b/packages/shared/src/protocol/payloads/lobbyPayloads.ts
@@ -4,6 +4,7 @@
  * ルーム参加前後の契約を共有する
  */
 import type * as roomTypes from "../../domains/room/room.type";
+export type { TeamAssignmentMode } from "../../domains/room/room.type";
 
 /** JOIN_ROOM イベントで送受信するルーム参加情報 */
 export type JoinRoomPayload = roomTypes.JoinRoomPayload;
@@ -18,4 +19,19 @@
 export type LobbySettingsUpdatePayload = {
   targetPlayerCount: number;
   fieldSizePreset: roomTypes.Room["fieldSizePreset"];
+  teamAssignmentMode: roomTypes.TeamAssignmentMode;
+};
+
+/** SELECT_TEAM イベントでプレイヤーが送信するチーム選択情報 */
+export type SelectTeamPayload = {
+  /** 選択したチームID（null = ランダム割り当て） */
+  preferredTeamId: number | null;
+};
+
+/** SELECT_TEAM_REJECTED イベントでサーバーが送信するチーム選択拒否情報 */
+export type SelectTeamRejectedPayload = {
+  /** 拒否されたチームID */
+  preferredTeamId: number;
+  /** 拒否理由 */
+  reason: "team_full";
 };
diff --git a/packages/shared/src/protocol/socketEvents.ts b/packages/shared/src/protocol/socketEvents.ts
index 85fe404..5e9d1fe 100644
--- a/packages/shared/src/protocol/socketEvents.ts
+++ b/packages/shared/src/protocol/socketEvents.ts
@@ -15,6 +15,8 @@
   ROOM_JOIN_REJECTED: "room-join-rejected",
   ROOM_UPDATE: "room-update",
   LOBBY_SETTINGS_UPDATE: "lobby-settings-update",
+  SELECT_TEAM: "select-team",
+  SELECT_TEAM_REJECTED: "select-team-rejected",
   START_GAME: "start-game",
   GAME_START: "game-start",
   READY_FOR_GAME: "ready-for-game",