A chair includes at least one leg; a rotatable seat; a backrest; and a headrest including a clamping member having a yoke with two hollow cylindrical yoke arms; two hook members extending forward downward from both sides of the yoke respectively; and a lower well; a support including a hinge head pivotably disposed between the yoke arms; and two opposite fastening portions; a head frame including two rear bars and two joints frictionally, slidably fastened on the bars, each joint frictionally, pivotably secured to the fastening portion; a hooking member in the well and including two hook elements curved upward forward wherein the hook elements cooperate with the hook members to releasably mount the headrest on the backrest; a first hinge element pivotably disposed in one yoke arm and includes a lever; and a second hinge element moveably disposed in the other yoke arm and the hinge head.