diff --git a/src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/domain/game/GameController.java b/src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/domain/game/GameController.java
index b90de4f..361879d 100644
--- a/src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/domain/game/GameController.java
+++ b/src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/domain/game/GameController.java
@@ -61,6 +61,35 @@ public class GameController {
         engine.getState().addPlayer(name, chips);
     }
 
+    /**
+     * Renames a player id in the controller list and underlying game state.
+     *
+     * @param oldId old player id
+     * @param newId new player id
+     * @return true if rename succeeded
+     */
+    public boolean renamePlayer(PlayerId oldId, PlayerId newId) {
+        if (oldId == null || newId == null) {
+            return false;
+        }
+        if (oldId.equals(newId)) {
+            return true;
+        }
+
+        int idx = players.indexOf(oldId);
+        if (idx < 0 || players.contains(newId)) {
+            return false;
+        }
+
+        boolean stateRenamed = engine.getState().renamePlayerId(oldId, newId);
+        if (!stateRenamed) {
+            return false;
+        }
+
+        players.set(idx, newId);
+        return true;
+    }
+
     /**
      * Initializes a new hand by preparing the deck, setting the phase to PREFLOP, rotating the
      * dealer, dealing hole cards, posting blinds, and setting the first active player.
diff --git a/src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/domain/game/player/Player.java b/src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/domain/game/player/Player.java
index f937e44..81576dd 100644
--- a/src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/domain/game/player/Player.java
+++ b/src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/domain/game/player/Player.java
@@ -49,6 +49,16 @@ public class Player {
         return id;
     }
 
+    /**
+     * Updates the player's id. This is used when a username change is propagated into an already
+     * running game.
+     *
+     * @param id new player id
+     */
+    public void setId(PlayerId id) {
+        this.id = id;
+    }
+
     /**
      * Returns the display name of the player. Currently identical to the player ID.
      *
diff --git a/src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/domain/game/state/GameState.java b/src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/domain/game/state/GameState.java
index a018d6a..1732072 100644
--- a/src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/domain/game/state/GameState.java
+++ b/src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/domain/game/state/GameState.java
@@ -174,6 +174,51 @@ public class GameState {
         holeCards.computeIfAbsent(id, k -> new ArrayList<>());
     }
 
+    /**
+     * Renames a player id across all game-state structures.
+     *
+     * @param oldId existing player id
+     * @param newId new player id
+     * @return true if the rename was applied, false otherwise
+     */
+    public synchronized boolean renamePlayerId(PlayerId oldId, PlayerId newId) {
+        if (oldId == null || newId == null) {
+            return false;
+        }
+        if (!players.containsKey(oldId)) {
+            return false;
+        }
+        if (oldId.equals(newId)) {
+            return true;
+        }
+        if (players.containsKey(newId)) {
+            return false;
+        }
+
+        Player player = players.remove(oldId);
+        if (player == null) {
+            return false;
+        }
+        player.setId(newId);
+        players.put(newId, player);
+
+        int idx = playerOrder.indexOf(oldId);
+        if (idx >= 0) {
+            playerOrder.set(idx, newId);
+        }
+
+        moveMapEntry(currentBets, oldId, newId, 0);
+        moveMapEntry(playerBetCommitments, oldId, newId, 0);
+        moveMapEntry(holeCards, oldId, newId, new ArrayList<>());
+        return true;
+    }
+
+    private <T> void moveMapEntry(
+            Map<PlayerId, T> map, PlayerId oldId, PlayerId newId, T fallback) {
+        T value = map.remove(oldId);
+        map.put(newId, value != null ? value : fallback);
+    }
+
     // Betting
     public int getCurrentBet(PlayerId playerId) {
         return currentBets.getOrDefault(playerId, 0);