The man's body was found at Contra Loma Regional Park after officers responded to an 11:20 p.m. call, East Bay Regional Park District police said.
Officers removed the man from the lake and gave him immediate medical assistance, but emergency rescue personnel pronounced the man dead at the scene, according to police.
The victim was in the Bay Area visiting family, police said.
Responding units included officers from the East Bay Regional Park District Police Department, Contra Costa County Sheriff's Office and the Antioch Police Department, according to police.
Investigators are asking that anyone who was in the area to contact the East Bay Regional Park District Police Department investigations unit at (510) 690-6549. To make an anonymous tip, call the East Bay Regional Park District confidential tip line at (510) 690-6521.