diff --git a/apps/client/src/scenes/game/application/network/handlers/GameNetworkStateApplier.ts b/apps/client/src/scenes/game/application/network/handlers/GameNetworkStateApplier.ts
index 1df634d..f38e05c 100644
--- a/apps/client/src/scenes/game/application/network/handlers/GameNetworkStateApplier.ts
+++ b/apps/client/src/scenes/game/application/network/handlers/GameNetworkStateApplier.ts
@@ -9,6 +9,7 @@
   BombPlacedPayload,
   PlayerDeadPayload,
 } from "@repo/shared";
+import { domain } from "@repo/shared";
 import { AppearanceResolver } from "@client/scenes/game/application/AppearanceResolver";
 import { GameMapController } from "@client/scenes/game/entities/map/GameMapController";
 import { PlayerRepository } from "@client/scenes/game/entities/player/PlayerRepository";
@@ -105,7 +106,8 @@
         this.playerSyncHandler.handleRemovePlayer(payload);
       },
       onReceivedUpdateMapCells: (payload) => {
-        this.mapSyncHandler.handleUpdateMapCells(payload);
+        const updates = domain.game.gridMap.ungroupCellUpdates(payload);
+        this.mapSyncHandler.handleUpdateMapCells(updates);
       },
       onReceivedGameEnd: () => {
         this.onGameEnded();
diff --git a/apps/client/src/scenes/game/application/network/handlers/MapSyncHandler.ts b/apps/client/src/scenes/game/application/network/handlers/MapSyncHandler.ts
index 2e5f871..c2dd86b 100644
--- a/apps/client/src/scenes/game/application/network/handlers/MapSyncHandler.ts
+++ b/apps/client/src/scenes/game/application/network/handlers/MapSyncHandler.ts
@@ -3,7 +3,7 @@
  * マップ同期イベントの受信処理を担当する
  * セル更新データをマップへ適用する
  */
-import type { UpdateMapCellsPayload } from "@repo/shared";
+import type { domain } from "@repo/shared";
 import { GameMapController } from "@client/scenes/game/entities/map/GameMapController";
 
 /** MapSyncHandler の初期化入力 */
@@ -20,7 +20,7 @@
   }
 
   /** マップセル更新を適用する */
-  public handleUpdateMapCells = (updates: UpdateMapCellsPayload): void => {
+  public handleUpdateMapCells = (updates: domain.game.gridMap.CellUpdate[]): void => {
     this.gameMap.updateCells(updates);
   };
 }
\ No newline at end of file
diff --git a/apps/server/src/domains/game/application/ports/gameUseCasePorts.ts b/apps/server/src/domains/game/application/ports/gameUseCasePorts.ts
index 2c08386..c071fea 100644
--- a/apps/server/src/domains/game/application/ports/gameUseCasePorts.ts
+++ b/apps/server/src/domains/game/application/ports/gameUseCasePorts.ts
@@ -14,7 +14,6 @@
   GameStartPayload,
   PongPayload,
   RemovePlayerPayload,
-  UpdateMapCellsPayload,
   UpdatePlayersPayload,
 } from "@repo/shared";
 import type { GameSessionCallbacks } from "../services/GameRoomSession";
@@ -55,7 +54,7 @@
   ): void;
   publishMapCellUpdatesToRoom(
     roomId: domain.room.Room["roomId"],
-    cellUpdates: UpdateMapCellsPayload,
+    cellUpdates: domain.game.gridMap.CellUpdate[],
   ): void;
   publishGameEndToRoom(roomId: domain.room.Room["roomId"]): void;
   publishGameResultToRoom(
diff --git a/apps/server/src/domains/game/entities/map/MapStore.ts b/apps/server/src/domains/game/entities/map/MapStore.ts
index bf09972..191417a 100644
--- a/apps/server/src/domains/game/entities/map/MapStore.ts
+++ b/apps/server/src/domains/game/entities/map/MapStore.ts
@@ -5,7 +5,6 @@
 import { domain } from "@repo/shared";
 import { createInitialGridColors } from "./mapGrid.js";
 import { paintCellIfChanged } from "./mapPainting.js";
-import { drainPendingUpdates } from "./mapUpdates.js";
 
 /** ルーム内マップの塗り状態と更新差分を管理するストア */
 export class MapStore {
@@ -34,9 +33,12 @@
 
   /**
     * 溜まっている差分を取得し，キューをクリアする（ループ送信時に使用）
+    * 参照をそのまま返却し新しい空配列で差し替えることでコピーを回避する
    */
   public getAndClearUpdates(): domain.game.gridMap.CellUpdate[] {
-    return drainPendingUpdates(this.pendingUpdates);
+    const updates = this.pendingUpdates;
+    this.pendingUpdates = [];
+    return updates;
   }
 
   /** 現在のマップ塗り状態をスナップショットとして返す */
diff --git a/apps/server/src/domains/game/entities/map/mapContestResolver.ts b/apps/server/src/domains/game/entities/map/mapContestResolver.ts
new file mode 100644
index 0000000..bc77662
--- /dev/null
+++ b/apps/server/src/domains/game/entities/map/mapContestResolver.ts
@@ -0,0 +1,47 @@
+/**
+ * mapContestResolver
+ * 同一セルに複数チームが重なった場合の競合判定を提供する
+ * 純粋関数として切り出し，単体テストを容易にする
+ */
+
+/** 同一セルに異なるチームが存在していることを示すセンチネル値 */
+export const CONTESTED_CELL = -2;
+
+/** プレイヤーごとのグリッド位置情報 */
+export type PlayerGridEntry = {
+  playerId: string;
+  gridIndex: number | null;
+  teamId: number;
+};
+
+/**
+ * 各セルの占有チームを判定し，競合セルは CONTESTED_CELL でマークする
+ * @returns セルインデックス → チームID（競合時は CONTESTED_CELL）のマップ
+ */
+export const resolveUncontestedCells = (
+  entries: PlayerGridEntry[],
+): Map<number, number> => {
+  const cellTeamMap = new Map<number, number>();
+
+  for (const { gridIndex, teamId } of entries) {
+    if (gridIndex === null) continue;
+
+    const existing = cellTeamMap.get(gridIndex);
+    if (existing === undefined) {
+      cellTeamMap.set(gridIndex, teamId);
+    } else if (existing !== CONTESTED_CELL && existing !== teamId) {
+      cellTeamMap.set(gridIndex, CONTESTED_CELL);
+    }
+  }
+
+  return cellTeamMap;
+};
+
+/** 指定セルが塗り可能（競合でない）かを判定する */
+export const isCellPaintable = (
+  cellTeamMap: Map<number, number>,
+  gridIndex: number,
+): boolean => {
+  const ownerTeamId = cellTeamMap.get(gridIndex);
+  return ownerTeamId !== undefined && ownerTeamId !== CONTESTED_CELL;
+};
diff --git a/apps/server/src/domains/game/entities/map/mapUpdates.ts b/apps/server/src/domains/game/entities/map/mapUpdates.ts
deleted file mode 100644
index 77ce63a..0000000
--- a/apps/server/src/domains/game/entities/map/mapUpdates.ts
+++ /dev/null
@@ -1,14 +0,0 @@
-/**
- * mapUpdates
- * マップ差分キューの取り出しとクリア処理を提供する
- */
-import { domain } from "@repo/shared";
-
-/** 差分キューを配列として返却し，キューを空にする */
-export const drainPendingUpdates = (
-  pendingUpdates: domain.game.gridMap.CellUpdate[]
-): domain.game.gridMap.CellUpdate[] => {
-  const updates = [...pendingUpdates];
-  pendingUpdates.length = 0;
-  return updates;
-};
diff --git a/apps/server/src/domains/game/loop/GameLoop.ts b/apps/server/src/domains/game/loop/GameLoop.ts
index e2471d1..a9c21ad 100644
--- a/apps/server/src/domains/game/loop/GameLoop.ts
+++ b/apps/server/src/domains/game/loop/GameLoop.ts
@@ -5,6 +5,11 @@
 import { Player } from "../entities/player/Player.js";
 import { MapStore } from "../entities/map/MapStore";
 import { getPlayerGridIndex } from "../entities/player/playerPosition.js";
+import {
+  resolveUncontestedCells,
+  isCellPaintable,
+  type PlayerGridEntry,
+} from "../entities/map/mapContestResolver.js";
 import { config } from "@server/config";
 import { domain } from "@repo/shared";
 import type { PlaceBombPayload } from "@repo/shared";
@@ -42,6 +47,9 @@
   onBotBombHit?: (targetPlayerId: string, bombId: string) => void;
 };
 
+/** プレイヤーのグリッド位置キャッシュを含むエントリ */
+type PlayerGridCacheEntry = PlayerGridEntry & { player: Player };
+
 /** ルーム内ゲーム進行を定周期で実行するループ管理クラス */
 export class GameLoop {
   private loopId: NodeJS.Timeout | null = null;
@@ -246,14 +254,24 @@
   ): domain.game.tick.TickData["playerUpdates"] {
     const changedPlayers: domain.game.tick.TickData["playerUpdates"] = [];
 
+    // 全プレイヤーのグリッド位置を1度だけ計算してキャッシュする
+    const gridEntries: PlayerGridCacheEntry[] = [];
     this.players.forEach((player) => {
-      activePlayerIds.add(player.id);
-      const gridIndex = getPlayerGridIndex(player);
-      if (gridIndex !== null) {
-        this.mapStore.paintCell(gridIndex, player.teamId);
-      }
+      gridEntries.push({
+        playerId: player.id,
+        gridIndex: getPlayerGridIndex(player),
+        teamId: player.teamId,
+        player,
+      });
+    });
 
-      // 送信用のプレイヤーデータを構築
+    // 競合判定を経てマップを塗る
+    this.paintUncontestedCells(gridEntries);
+
+    // プレイヤー差分を収集する
+    for (const { playerId, player } of gridEntries) {
+      activePlayerIds.add(playerId);
+
       const playerData: domain.game.tick.PlayerPositionUpdate = {
         id: player.id,
         x: player.x,
@@ -270,11 +288,22 @@
         changedPlayers.push(playerData);
         this.lastSentPlayers.set(player.id, playerData);
       }
-    });
+    }
 
     return changedPlayers;
   }
 
+  /** 競合判定を行い，単一チームが占有するセルのみを塗る */
+  private paintUncontestedCells(gridEntries: PlayerGridCacheEntry[]): void {
+    const cellTeamMap = resolveUncontestedCells(gridEntries);
+
+    for (const { gridIndex, player } of gridEntries) {
+      if (gridIndex !== null && isCellPaintable(cellTeamMap, gridIndex)) {
+        this.mapStore.paintCell(gridIndex, player.teamId);
+      }
+    }
+  }
+
   private cleanupInactivePlayerSnapshots(activePlayerIds: Set<string>): void {
     Array.from(this.lastSentPlayers.keys()).forEach((playerId) => {
       if (!activePlayerIds.has(playerId)) {
diff --git a/apps/server/src/network/handlers/game/createGameOutputAdapter.ts b/apps/server/src/network/handlers/game/createGameOutputAdapter.ts
index f897f63..c8f85b3 100644
--- a/apps/server/src/network/handlers/game/createGameOutputAdapter.ts
+++ b/apps/server/src/network/handlers/game/createGameOutputAdapter.ts
@@ -3,7 +3,7 @@
  * ゲーム系ユースケースから利用する送信関数群を生成する
  */
 import { Server } from "socket.io";
-import { contracts as protocol } from "@repo/shared";
+import { contracts as protocol, domain as domainNs } from "@repo/shared";
 import type {
   BombPlacedAckPayload,
   BombPlacedPayload,
@@ -14,7 +14,6 @@
   PongPayload,
   CurrentPlayersPayload,
   RemovePlayerPayload,
-  UpdateMapCellsPayload,
   UpdatePlayersPayload,
 } from "@repo/shared";
 import type {
@@ -64,12 +63,13 @@
     },
     publishMapCellUpdatesToRoom: (
       roomId: RoomId,
-      cellUpdates: UpdateMapCellsPayload,
+      cellUpdates: domainNs.game.gridMap.CellUpdate[],
     ) => {
+      const grouped = domainNs.game.gridMap.groupCellUpdates(cellUpdates);
       common.emitToRoom(
         roomId,
         protocol.SocketEvents.UPDATE_MAP_CELLS,
-        cellUpdates,
+        grouped,
       );
     },
     publishGameEndToRoom: (roomId: RoomId) => {
diff --git a/packages/shared/src/domains/game/gridMap/gridMap.type.ts b/packages/shared/src/domains/game/gridMap/gridMap.type.ts
index c59a2da..327c6cd 100644
--- a/packages/shared/src/domains/game/gridMap/gridMap.type.ts
+++ b/packages/shared/src/domains/game/gridMap/gridMap.type.ts
@@ -14,3 +14,10 @@
   index: number;
   teamId: number;
 }
+
+/**
+ * teamId別にグループ化したマップ差分更新
+ * キーは teamId（文字列），値は塗り替え対象セルの index 配列
+ * 帯域最適化のためキー名の重複を排除した形式で送受信する
+ */
+export type GroupedCellUpdates = Record<string, number[]>;
diff --git a/packages/shared/src/domains/game/gridMap/groupedCellUpdates.ts b/packages/shared/src/domains/game/gridMap/groupedCellUpdates.ts
new file mode 100644
index 0000000..758cd87
--- /dev/null
+++ b/packages/shared/src/domains/game/gridMap/groupedCellUpdates.ts
@@ -0,0 +1,43 @@
+/**
+ * groupedCellUpdates
+ * CellUpdate配列とGroupedCellUpdatesの相互変換ユーティリティを提供する
+ * サーバー送信時のグループ化とクライアント受信時の展開を担う
+ */
+import type { CellUpdate, GroupedCellUpdates } from "./gridMap.type";
+
+/** CellUpdate 配列を teamId 別にグループ化する（送信時） */
+export const groupCellUpdates = (
+  updates: CellUpdate[],
+): GroupedCellUpdates => {
+  const grouped: GroupedCellUpdates = {};
+
+  for (const { index, teamId } of updates) {
+    const key = String(teamId);
+    const list = grouped[key];
+
+    if (list) {
+      list.push(index);
+    } else {
+      grouped[key] = [index];
+    }
+  }
+
+  return grouped;
+};
+
+/** GroupedCellUpdates を CellUpdate 配列へ展開する（受信時） */
+export const ungroupCellUpdates = (
+  grouped: GroupedCellUpdates,
+): CellUpdate[] => {
+  const updates: CellUpdate[] = [];
+
+  for (const [teamIdStr, indices] of Object.entries(grouped)) {
+    const teamId = Number(teamIdStr);
+
+    for (const index of indices) {
+      updates.push({ index, teamId });
+    }
+  }
+
+  return updates;
+};
diff --git a/packages/shared/src/domains/game/gridMap/index.ts b/packages/shared/src/domains/game/gridMap/index.ts
index cef8bbc..af9db0a 100644
--- a/packages/shared/src/domains/game/gridMap/index.ts
+++ b/packages/shared/src/domains/game/gridMap/index.ts
@@ -5,6 +5,8 @@
  */
 
 /** グリッドマップ関連の型を再公開する */
-export type { MapState, CellUpdate } from "./gridMap.type";
+export type { MapState, CellUpdate, GroupedCellUpdates } from "./gridMap.type";
 /** グリッド座標変換ロジックを再公開する */
 export { getGridIndexFromPosition } from "./gridMap.logic";
+/** CellUpdate配列とGroupedCellUpdatesの相互変換を再公開する */
+export { groupCellUpdates, ungroupCellUpdates } from "./groupedCellUpdates";
diff --git a/packages/shared/src/protocol/payloads/gamePayloads.ts b/packages/shared/src/protocol/payloads/gamePayloads.ts
index 7090983..80752ad 100644
--- a/packages/shared/src/protocol/payloads/gamePayloads.ts
+++ b/packages/shared/src/protocol/payloads/gamePayloads.ts
@@ -4,7 +4,7 @@
  * プレイヤー差分，マップ差分，開始終了系の契約を集約する
  */
 import type { PlayerPositionUpdate } from "../../domains/game/tick/tick.type";
-import type { CellUpdate } from "../../domains/game/gridMap/gridMap.type";
+import type { GroupedCellUpdates } from "../../domains/game/gridMap/gridMap.type";
 import type {
   MovePayload as PlayerMovePayload,
   PlayerData,
@@ -52,8 +52,8 @@
 /** current-players イベントで送受信するプレイヤー一覧 */
 export type CurrentPlayersPayload = PlayerSnapshotPayload;
 
-/** update-map-cells イベントで送受信するマップ差分配列 */
-export type UpdateMapCellsPayload = CellUpdate[];
+/** update-map-cells イベントで送受信するグループ化マップ差分 */
+export type UpdateMapCellsPayload = GroupedCellUpdates;
 
 /**
  * new-player イベントで送受信するプレイヤー情報