The 2-year-old boy was reported missing from his family's home in Williamstown, New Jersey at 8:23 p.m.
His aunt called police after finding the front door of the home open.
Responding to the aunt's call were members of the Monroe Township police and fire departments.
Police say the aunt, who is in her late 20s, and a friend found the boy in Victory Lake about 100 yards from the 12th Avenue home.
The boy, described to investigators as autistic, was in about three feet of water.
The 2-year-old was pronounced dead at Kennedy Memorial Hospital in Washington Township shortly after 11:00 p.m. after almost two hours on a life-support system.
The incident remains under investigation by the Gloucester County Prosecutor's Office and the Monroe Township Police Department.