Abstract: An air-driven post driver that includes a hammer assembly and a post receiving assembly. The hammer assembly includes an air-driven piston hammer slidably disposed within a cylinder bore of a cylinder body having a circumferential compressed air groove of a first groove width formed into the cylinder wall that defines the cylinder bore and an upper discharge port and a lower discharge port. The piston hammer includes an upward thrusting air passageway and a downward thrusting air passageway. The upward thrusting air passageway includes a radially oriented upward intake passageway and a longitudinally oriented upward discharge passageway. The upward discharge passageway is of a length greater than one half the length of the piston hammer. The downward thrusting passageway includes a radially oriented downward intake passageway and a longitudinally oriented downward discharge passageway having a threaded downward discharge port.