The inmate who is serving an 18-month sentence for possession of a knife, was found by wardens on Sunday morning.
Prison officials said the incident was "extremely serious".
The Ministry of Justice said the man, who was not on suicide watch, had since been returned to the prison, after receiving treatment in hospital.
A prison service spokeswoman said: "We can confirm a prisoner at HMP Bristol has seriously self-harmed.
"The prisoner was treated in hospital and has now returned to the prison."