An armrest assembly for a chair includes an armrest support, a mounting sleeve mounted on the armrest support, an armrest mounted on the mounting sleeve, and a lifting device mounted between the armrest support and the mounting sleeve to lock or unlock the mounting sleeve onto or from the armrest support. Thus, the mounting sleeve is movable relative to the armrest support so as to adjust the height of the armrest so that the height of the armrest can be adjusted freely to support a user's arm comfortably. In addition, a user only needs to pull the armrest upward to adjust the height of the armrest so that the user can adjust the height of the armrest easily and quickly in an energy-saving manner.