The assault happened between 9:20 p.m. and 9:45 p.m. in a common area inside the jail, located at 5555 Giant Highway, Lee said.
Staff members found the inmate, a 47-year-old Concord man, suffering from numerous injuries. He was airlifted to John Muir Medical Center in Walnut Creek. His name has not yet been released.
The man had been arrested by Richmond police on suspicion of burglary, Lee said.
As of this morning, investigators were still working to determine a motive for the attack and identify suspects.