An adjustable chair includes a chair base, a backrest frame, a backrest plate and two armrest structures. The backrest frame is pivotally disposed on the chair base, wherein the backrest frame includes two side panels. The backrest plate is movably disposed on the backrest frame. The two armrest structures are respectively pivotally connected to the two side panels, and each of the two armrest structures includes a fixing plate, a pin, a stabilizing element and an armrest bracket. The fixing plate is fixedly disposed on one of the two side panels and includes a pivoted hole. The pin element passes through and is disposed in the pivoted hole. The pin element is disposed in the stabilizing element. The armrest bracket is locked and connected to the pin element.