The man had been arrested on 8th May outside a Bristol nightclub. He was taken to the custody suite on Ashmead Road and booked in.
The IPCC said: “At approximately 2am the man was involved in an incident with a custody sergeant during which he suffered a serious head injury.
“He was taken to Southmead Hospital where it was confirmed that he had a fractured skull, bleeding on the brain and a perforated ear drum. He has subsequently been released from hospital.
“The sergeant has been served with a gross misconduct notice and has been interviewed under criminal caution on suspicion of unlawfully wounding/inflicting grievous bodily harm.”