The Reds placed Williamson on the 15-day injured list Wednesday with a left elbow strain.
Williamson exited his start Tuesday against Atlanta because of the injury, and it has now officially ended his season. The left-hander also missed much of the campaign with a Bennett lesion in his left shoulder, ultimately opting to treat that injury with injections rather than surgery. It's not clear at this point whether an offseason operation for either injury might be on the table.