An adjustable armrest structure for a chair is an adjusting assembly fixed to bottom ends of two armrests on two sides of the chair and includes a body, two side rails, and a movable member. The body includes a guiding slot formed on a bottom end thereof to fit a screwing element and to engage with the movable member. The guiding slot of the body includes a plurality of arcuate recesses arranged around the two sides thereof to fit two projections fixed on two sides of a guide member respectively. An actuating member includes a bore disposed thereon and is fixed to the guide member. The screwing element is inserted through the bore of the actuating member and an orifice of the guide member. A screw segment of the screwing element is screwed with the aperture of the movable member.