Rex Clark (14 September 1935 — 19 October 1978) was an officer of the Australian Army and a collector of orders, decorations and medals as well as military history books and ephemera. Clark gained notoriety due to his service as a mercenary in foreign wars while on leave from the Australian Army. He was appointed Officer of the Order of Saint John and received Imperial and Australian awards for Vietnam service and long service. He was awarded a number of foreign awards although during his lifetime such awards were not officially recognised by Australia. He ended his own life while being investigated in relation to the disappearance of militaria from museums and private collections.