We conduct a comprehensive study of the Amorgos, Greece earthquake and tsunami of 1956 July 09, the largest such event in the Aegean Sea in the 20th century. Systematic relocation of the main shock and 34 associated events defines a rupture area measuring 75×40 km. The use of the Preliminary Determination of Focal Mechanism algorithm resolves the longstanding controversy about the focal geometry of the event, yielding a normal faulting mechanism along a plane dipping to the southeast, which expresses extensional tectonics in the back arc behind the Hellenic subduction zone. The seismic moment of 3.9×1027 dyn cm is the largest measured in the past 100 yr in the Mediterranean Basin.