A foldable chair has an armrest retracting structure that includes two plate elements placed below each armrest, each plate element having a guide slot with a linear section and a reverse turn section. A first pivotal axle passes through the plate elements and connects with the front leg frame. A support beam has an end through which a second pivotal axle is mounted, the second pivotal axle being connected with the front leg frame. An opposite end of the support beam is secured in the slots through a bolt. By moving the support beam, the armrest can be deployed or retracted. When the bolt is lodged in the reverse turn section, the armrest can be locked in the deployed position.