Police were called to the gym on the 100 block of El Camino Real in South San Francisco around 9:30 p.m. Wednesday on a report of an assault.
Witnesses told officers that a gym member, later identified as South San Francisco resident Kenneth Osako, 46, had attacked another gym member with a solid steel bar normally used to lift weights.
Osako, a plumber, allegedly struck the man in the head several times, then dropped the steel bar and left the scene, police said.
The victim was taken to San Francisco General Hospital, where he died Thursday of his injuries.
Osako was located Thursday morning and arrested by South San Francisco police and a special agent from the Department of Homeland Security, police said.
He has been booked into San Mateo County jail on suspicion of murder.