diff --git a/src/app/api/conversations/join/route.js b/src/app/api/conversations/join/route.js
index 2f71ee5..570a127 100644
--- a/src/app/api/conversations/join/route.js
+++ b/src/app/api/conversations/join/route.js
@@ -36,6 +36,18 @@ export const POST = withAuth(async ({ request, locals }) => {
 
 		const userId = userData.id;
 
+		const { data: participant, error: participantError } = await supabase
+			.from('conversation_participants')
+			.select('id')
+			.eq('conversation_id', conversationId)
+			.eq('user_id', userId)
+			.is('left_at', null)
+			.single();
+
+		if (participantError || !participant) {
+			return NextResponse.json({ error: 'Access denied to conversation' }, { status: 403 });
+		}
+
 		// Join SSE room for real-time updates
 		sseManager.joinRoom(userId, conversationId);
 
diff --git a/src/app/api/conversations/join/route.test.js b/src/app/api/conversations/join/route.test.js
index 3f6ac00..e5fdf51 100644
--- a/src/app/api/conversations/join/route.test.js
+++ b/src/app/api/conversations/join/route.test.js
@@ -5,7 +5,10 @@ const mocks = vi.hoisted(() => ({
 		from: vi.fn()
 	},
 	mockJoinRoom: vi.fn(),
-	userEq: vi.fn()
+	userEq: vi.fn(),
+	participantEq: vi.fn(),
+	participantIs: vi.fn(),
+	participantSingle: vi.fn()
 }));
 
 vi.mock('@/lib/api/middleware/auth.js', () => ({
@@ -39,6 +42,18 @@ function createUserQuery() {
 	return query;
 }
 
+function createParticipantQuery() {
+	const query = {
+		select: vi.fn(() => query),
+		eq: mocks.participantEq,
+		is: mocks.participantIs,
+		single: mocks.participantSingle
+	};
+	mocks.participantEq.mockReturnValue(query);
+	mocks.participantIs.mockReturnValue(query);
+	return query;
+}
+
 function createRequest(conversationId) {
 	return {
 		json: vi.fn().mockResolvedValue({ conversationId })
@@ -49,8 +64,13 @@ describe('POST /api/conversations/join', () => {
 	beforeEach(() => {
 		vi.resetModules();
 		vi.clearAllMocks();
+		mocks.participantSingle.mockResolvedValue({
+			data: { id: 'participant-id' },
+			error: null
+		});
 		mocks.mockSupabase.from.mockImplementation((table) => {
 			if (table === 'users') return createUserQuery();
+			if (table === 'conversation_participants') return createParticipantQuery();
 			throw new Error(`Unexpected table: ${table}`);
 		});
 	});
@@ -72,6 +92,24 @@ describe('POST /api/conversations/join', () => {
 
 		expect(response.status).toBe(200);
 		expect(mocks.userEq).toHaveBeenCalledWith('auth_user_id', 'auth-user-id');
+		expect(mocks.participantEq).toHaveBeenCalledWith('conversation_id', 'conversation-1');
+		expect(mocks.participantEq).toHaveBeenCalledWith('user_id', 'internal-user-id');
+		expect(mocks.participantIs).toHaveBeenCalledWith('left_at', null);
 		expect(mocks.mockJoinRoom).toHaveBeenCalledWith('internal-user-id', 'conversation-1');
 	});
+
+	it('rejects users who are not active conversation participants', async () => {
+		mocks.participantSingle.mockResolvedValue({
+			data: null,
+			error: { message: 'No rows found' }
+		});
+
+		const { POST } = await import('./route.js');
+		const response = await POST(createRequest('conversation-1'));
+		const body = await response.json();
+
+		expect(response.status).toBe(403);
+		expect(body).toEqual({ error: 'Access denied to conversation' });
+		expect(mocks.mockJoinRoom).not.toHaveBeenCalled();
+	});
 });