A garment clamp apparatus for stabilizing a shirt during pressing of the sleeves. The apparatus is for use on a shirt pressing machine of the type having a housing, a plurality of press platens supported by the housing, and a movable tray carrying sleeve supports or bucks. The garment clamp apparatus includes a clamp for securely mounting the apparatus to the support structure of the housing. A frame extends forwardly from the clamp between the press platens. A pneumatic cylinder connects the frame with an extendable foot, which is movable linearly against the bias of a pair of return springs between a retracted position and an extended position. In the extended position, the foot engages the shirt in the area between the sleeves and holds the sleeves taut on the sleeve supports for the pressing operation. Guide shaft assemblies are disposed between the frame and the foot. The pneumatic cylinder can be connected into the existing pneumatic control system without modification of the control algorithm.