diff --git a/src/__tests__/GameStateManager.test.ts b/src/__tests__/GameStateManager.test.ts
index 402ca69..4817019 100644
--- a/src/__tests__/GameStateManager.test.ts
+++ b/src/__tests__/GameStateManager.test.ts
@@ -1,7 +1,8 @@
 // src/__tests__/GameStateManager.test.ts - Unit tests for GameStateManager class
 import { describe, test, expect, beforeEach, vi } from 'vitest';
-import { GameStateManager } from '../state/GameStateManager';
+import { GameStateManager, GameState } from '../state/GameStateManager';
 import { TypedEventEmitter } from '../game/EventEmitter';
+import type { GameEvents } from '../types';
 
 // Mock TypedEventEmitter for testing
 vi.mock('../game/EventEmitter', () => ({
@@ -9,64 +10,95 @@ vi.mock('../game/EventEmitter', () => ({
     on = vi.fn();
     emit = vi.fn();
     off = vi.fn();
+    removeAllListeners = vi.fn();
   },
 }));
 
 describe('GameStateManager', () => {
-  let mockEventEmitter: TypedEventEmitter<any>;
+  let mockEventEmitter: TypedEventEmitter<GameEvents>;
   let stateManager: GameStateManager;
 
   beforeEach(() => {
     // Create fresh mock event emitter for each test
     mockEventEmitter = new TypedEventEmitter() as any;
     stateManager = new GameStateManager(mockEventEmitter);
+    vi.clearAllMocks();
   });
 
   describe('initialization', () => {
     test('should initialize in IDLE state', () => {
-      // TODO: Implement test
-      expect(true).toBe(true);
+      expect(stateManager.getState()).toBe(GameState.IDLE);
     });
   });
 
   describe('state transitions', () => {
     test('should validate state transitions correctly', () => {
-      // TODO: Implement test
-      expect(true).toBe(true);
+      // Valid transition: IDLE → SELECTING
+      expect(stateManager.transitionTo(GameState.SELECTING)).toBe(true);
+      expect(stateManager.getState()).toBe(GameState.SELECTING);
+
+      // Valid transition: SELECTING → MATCHING
+      expect(stateManager.transitionTo(GameState.MATCHING)).toBe(true);
+      expect(stateManager.getState()).toBe(GameState.MATCHING);
     });
 
     test('should emit state change events on valid transition', () => {
-      // TODO: Implement test
-      expect(true).toBe(true);
+      stateManager.transitionTo(GameState.SELECTING);
+
+      expect(mockEventEmitter.emit).toHaveBeenCalledWith('game:stateChange', {
+        from: GameState.IDLE,
+        to: GameState.SELECTING
+      });
     });
 
     test('should return false for invalid transitions', () => {
-      // TODO: Implement test
-      expect(true).toBe(true);
+      // Invalid: IDLE → GAME_OVER (must go through MATCHING first)
+      expect(stateManager.transitionTo(GameState.GAME_OVER)).toBe(false);
+      expect(stateManager.getState()).toBe(GameState.IDLE);
+
+      // Invalid: SELECTING → IDLE is valid
+      expect(stateManager.transitionTo(GameState.SELECTING)).toBe(true);
+      expect(stateManager.transitionTo(GameState.IDLE)).toBe(true);
     });
   });
 
   describe('tile selection by state', () => {
     test('should allow tile selection in IDLE state', () => {
-      // TODO: Implement test
-      expect(true).toBe(true);
+      expect(stateManager.canSelectTile()).toBe(true);
+    });
+
+    test('should allow tile selection in SELECTING state', () => {
+      stateManager.transitionTo(GameState.SELECTING);
+      expect(stateManager.canSelectTile()).toBe(true);
     });
 
     test('should block tile selection in MATCHING state', () => {
-      // TODO: Implement test
-      expect(true).toBe(true);
+      stateManager.transitionTo(GameState.SELECTING);
+      stateManager.transitionTo(GameState.MATCHING);
+      expect(stateManager.canSelectTile()).toBe(false);
     });
 
     test('should block tile selection in GAME_OVER state', () => {
-      // TODO: Implement test
-      expect(true).toBe(true);
+      stateManager.transitionTo(GameState.SELECTING);
+      stateManager.transitionTo(GameState.MATCHING);
+      stateManager.transitionTo(GameState.GAME_OVER);
+      expect(stateManager.canSelectTile()).toBe(false);
     });
   });
 
   describe('game reset', () => {
     test('should reset from GAME_OVER to IDLE', () => {
-      // TODO: Implement test
-      expect(true).toBe(true);
+      stateManager.transitionTo(GameState.SELECTING);
+      stateManager.transitionTo(GameState.MATCHING);
+      stateManager.transitionTo(GameState.GAME_OVER);
+
+      stateManager.reset();
+
+      expect(stateManager.getState()).toBe(GameState.IDLE);
+      expect(mockEventEmitter.emit).toHaveBeenCalledWith('game:stateChange', {
+        from: GameState.GAME_OVER,
+        to: GameState.IDLE
+      });
     });
   });
 });
diff --git a/src/state/GameStateManager.ts b/src/state/GameStateManager.ts
index 714e5eb..90ccb78 100644
--- a/src/state/GameStateManager.ts
+++ b/src/state/GameStateManager.ts
@@ -1,35 +1,98 @@
-// src/state/GameStateManager.ts - Stub implementation for TDD
-// This will be implemented in Plan 04-01
-
+// src/state/GameStateManager.ts - Game state machine with transition validation
 import { TypedEventEmitter } from '../game/EventEmitter';
+import type { GameEvents } from '../types';
 
+/**
+ * Represents a game state in the state machine
+ * String enum for better debugging and logging
+ */
 export enum GameState {
+  /** Waiting for player input */
   IDLE = 'IDLE',
+  /** One tile selected, waiting for second tile */
   SELECTING = 'SELECTING',
+  /** Processing match, input blocked */
   MATCHING = 'MATCHING',
+  /** Game ended (win or no moves) */
   GAME_OVER = 'GAME_OVER',
 }
 
+/**
+ * Manages game state transitions with validation and event emission
+ * Enforces valid state transitions and provides helper methods for state-based logic
+ */
 export class GameStateManager {
-  private currentState: GameState = GameState.IDLE;
+  private currentState: GameState;
+  private readonly events: TypedEventEmitter<GameEvents>;
 
-  constructor(private events: TypedEventEmitter<any>) {}
+  /**
+   * Valid state transitions
+   * Maps each state to the list of states it can transition to
+   */
+  private readonly validTransitions: Record<GameState, GameState[]> = {
+    [GameState.IDLE]: [GameState.SELECTING],
+    [GameState.SELECTING]: [GameState.IDLE, GameState.MATCHING],
+    [GameState.MATCHING]: [GameState.IDLE, GameState.GAME_OVER],
+    [GameState.GAME_OVER]: [GameState.IDLE], // restart only
+  };
 
-  getCurrentState(): GameState {
+  constructor(events: TypedEventEmitter<GameEvents>) {
+    this.events = events;
+    this.currentState = GameState.IDLE;
+  }
+
+  /**
+   * Transition to a new state if valid
+   * @param newState - The state to transition to
+   * @returns true if transition succeeded, false if invalid
+   */
+  transitionTo(newState: GameState): boolean {
+    // Validate transition
+    const allowedStates = this.validTransitions[this.currentState];
+    if (!allowedStates.includes(newState)) {
+      return false;
+    }
+
+    // Perform transition
+    const previousState = this.currentState;
+    this.currentState = newState;
+
+    // Emit event for other components to react
+    this.events.emit('game:stateChange', {
+      from: previousState,
+      to: newState
+    });
+
+    return true;
+  }
+
+  /**
+   * Get the current game state
+   * @returns The current GameState
+   */
+  getState(): GameState {
     return this.currentState;
   }
 
+  /**
+   * Check if tiles can be selected in the current state
+   * @returns true if selection is allowed, false otherwise
+   */
   canSelectTile(): boolean {
     return this.currentState === GameState.IDLE || this.currentState === GameState.SELECTING;
   }
 
-  transitionTo(newState: GameState): boolean {
-    // TODO: Implement state transition validation
-    this.currentState = newState;
-    return true;
-  }
-
+  /**
+   * Reset the state machine to IDLE (for restart functionality)
+   * Emits a state change event from current state to IDLE
+   */
   reset(): void {
+    const previousState = this.currentState;
     this.currentState = GameState.IDLE;
+
+    this.events.emit('game:stateChange', {
+      from: previousState,
+      to: GameState.IDLE
+    });
   }
 }