diff --git a/src/client/ClientGameRunner.ts b/src/client/ClientGameRunner.ts
index c3c093426d..3585d051e0 100644
--- a/src/client/ClientGameRunner.ts
+++ b/src/client/ClientGameRunner.ts
@@ -69,7 +69,7 @@ export function joinLobby(
   lobbyConfig: LobbyConfig,
   onPrestart: () => void,
   onJoin: () => void,
-): () => void {
+): () => boolean {
   console.log(
     `joining lobby: gameID: ${lobbyConfig.gameID}, clientID: ${lobbyConfig.clientID}`,
   );
@@ -79,6 +79,8 @@ export function joinLobby(
 
   const transport = new Transport(lobbyConfig, eventBus);
 
+  let currentGameRunner: ClientGameRunner | null = null;
+
   const onconnect = () => {
     console.log(`Joined game lobby ${lobbyConfig.gameID}`);
     transport.joinGame(0);
@@ -113,7 +115,10 @@ export function joinLobby(
         userSettings,
         terrainLoad,
         terrainMapFileLoader,
-      ).then((r) => r.start());
+      ).then((r) => {
+        currentGameRunner = r;
+        r.start();
+      });
     }
     if (message.type === "error") {
       if (message.error === "full-lobby") {
@@ -139,8 +144,13 @@ export function joinLobby(
   };
   transport.connect(onconnect, onmessage);
   return () => {
+    if (currentGameRunner && currentGameRunner.shouldPreventWindowClose()) {
+      return false;
+    }
     console.log("leaving game");
+    currentGameRunner = null;
     transport.leaveGame();
+    return true;
   };
 }
 
@@ -230,6 +240,24 @@ export class ClientGameRunner {
     this.lastMessageTime = Date.now();
   }
 
+  /**
+   * Determines whether window closing should be prevented.
+   *
+   * Used to show a confirmation dialog when the user attempts to close
+   * the window or navigate away during an active game session.
+   *
+   * @returns {boolean} `true` if the window close should be prevented
+   * (when the player is alive in the game), `false` otherwise
+   * (when the player is not alive or doesn't exist)
+   */
+  public shouldPreventWindowClose(): boolean {
+    // Show confirmation dialog if player is alive in the game
+    if (this.myPlayer && this.myPlayer.isAlive()) {
+      return true;
+    }
+    return false;
+  }
+
   private saveGame(update: WinUpdate) {
     if (this.myPlayer === null) {
       return;
diff --git a/src/client/Main.ts b/src/client/Main.ts
index 6425c137f4..accaf9ea62 100644
--- a/src/client/Main.ts
+++ b/src/client/Main.ts
@@ -89,7 +89,7 @@ export interface JoinLobbyEvent {
 }
 
 class Client {
-  private gameStop: (() => void) | null = null;
+  private gameStop: (() => boolean) | null = null;
   private eventBus: EventBus = new EventBus();
 
   private usernameInput: UsernameInput | null = null;
@@ -150,11 +150,13 @@ class Client {
 
     this.publicLobby = document.querySelector("public-lobby") as PublicLobby;
 
-    window.addEventListener("beforeunload", () => {
-      console.log("Browser is closing");
-      if (this.gameStop !== null) {
-        this.gameStop();
+    window.addEventListener("beforeunload", (e) => {
+      if (this.gameStop && !this.gameStop()) {
+        e.preventDefault();
+        e.returnValue = "";
+        return "";
       }
+      console.log("Browser is closing");
     });
 
     const gutterAds = document.querySelector("gutter-ads");