A prisoner cut off his penis and attempted to flush it down the toilet.
The man was found bleeding by prison staff following the incident at HMP Bristol on Friday.
Paramedics rushed to the facility to treat the prisoner before taking him to hospital.
No other inmates were involved, the Prison Service said.
A spokesman said: "A prisoner from HMP Bristol suffered injuries during an incident on Friday. Paramedics were called and the offender was taken to hospital for further treatment."
The South Western Ambulance Service NHS Foundation Trust confirmed its staff were called to the prison shortly after 10am on Friday to reports of a male suffering "serious bleeding". A spokesman added: "Our paramedics stemmed the bleeding."
There was a similar incident at the facility in 2007 when a prisoner cut off his penis with a razorblade while locked in a cell.
He suffered deep laceration wounds before he was found by wardens.
HMP Bristol holds mainly adult males and is a category B facility, with the majority of inmates staying for less than a year.