Sources tell TMZ Fonda was driving on Sunset Blvd. in the Pacific Palisades area of Los Angeles at around 2 PM PT, when he saw a car parked on the side of the road.
Fonda got out of his car to check out the situation, and found the body of a man slumped over in the front seat.
Fonda called 911 to report his find. Cops and the coroner are now on scene.
Responders tell TMZ the man in the car took his own life ... and that the body has been there for about 3 days.