The man, whose name was not reported, dropped his keys in the manhole and became stuck when he tried to retrieve them, the Detroit Free Press reported.
He was taken by ambulance to a Detroit hospital, where he was pronounced dead, Detroit Police spokeswoman Sgt. Eren Stephens said.
"It appears it may have just been an unfortunate accident," she said.
Stephens said it was unclear if the manhole was covered. The Wayne County Medical Examiner's Office is scheduled to perform an autopsy on the man to determine the cause of death.