It was discovered in a parking area in Lower Bannister Street, Southampton, Hampshire, on Saturday morning.
Officers said the man lost the finger while trying to climb a fence and they were concerned he might have lost a lot of blood.
A Hampshire police spokesman said: "Investigations have found a man lost the finger in an accident while trying to climb a fence after getting trapped in a courtyard area.
"However he wandered off after being given a towel by a resident."
The spokesman said the man was later found after he had attended hospital for treatment.
He said: "We are pleased to say that the man who lost part of his finger in Southampton has now been traced. The 28-year-old is receiving treatment at hospital in Salisbury for his injury after seeking medical assistance himself."