Doctors at Royal Wigan Infirmary in Greater Manchester put out the alert after fearing the man faced amputation as the ring cut off his blood supply.
Two firefighters used a mini hand grinder to cut through the ring during a 20-minute procedure.
It is understood the man, aged in his 40s, was given an anaesthetic.
The firefighters placed a thin sheet of metal around his penis to protect the skin while removing the ring, which appeared to have been cut off from the end of a pipe.
Greater Manchester Fire and Rescue Service confirmed fire crews were called to the hospital at around 12.10 GMT on Thursday to "deal with a situation".
A spokeswoman for Royal Wigan Infirmary said they were unable to comment about the incident.