The invention relates to an assembly tool for assembling a sealing ring (3) in a ring groove (4, 9) of a machine component (1, 7), especially a valve disc (7). The aim of the invention is to produce the assembly tool in a simple and cost-effective manner. The assembly tool (10, 23) comprises a pneumatic piston-cylinder unit (11, 12, 26, 27) for evenly pressing the sealing ring (3) into the ring groove (4, 9). The inventive assembly tool is versatile, can be used in a mobile manner and enables to reliably assemble sealing components in such a way that nobody is injured.