An armrest assembly includes a base, an arm support, and a connector. The base includes a connecting portion and an installing portion formed at two opposite ends thereof. The installing portion is adapted for installing to a chair. A first surface at one end of the connecting portion is opposite to the installing portion. The arm support is supported on the base and includes a second surface faced to the first surface of the base. A cavity is defined in one of the first and second surfaces. An engaging member is formed on the other of the first and second surfaces and pivotably received in the cavity between an unlocked position and a locked position. A connector is received in the cavity of the arm support and engaged with the engaging member. The arm support is fixed to the base in the locked position.