An adjustable armrest for a chair is disclosed herein and comprises an assembly of a mounting plate, a support column, an anti-rotation feature, and two arms. The adjustable armrest's mounting plate is positionable laterally and rotationally about the support column when the anti-rotation feature is disengaged, and is restrained when the anti-rotation feature is engaged. The anti-rotation feature restrains the position of the mounting plate by restraining the rotation of the two arms and is preferably controlled via an actuator button.