A height-adjustable handrest has constituent components describe below. A middle bar element is inserted into a handrest external bar and has an internal space for containing a spring and an inner bar element. The handrest external bar is enclosed by a support unit with an upper end pivotally coupled to a press element. The press element has one end abutting against the inner bar element and has the other end protruding under an upward thrust from the spring. A pin passing through slots formed at the handrest external bar to penetrate oblong holes disposed at the inner bar element and the middle bar element, respectively. When pressed, the press element drives the inner bar element to move downward, thereby moving the pin upward along the inclined oblong hole of the inner bar element, compressing the spring downward, and moving the pin into a longitudinal channel of the handrest external bar.