It happened around 6pm Saturday on the North Freeway near Coronado.
Investigators say two children saw a Chevrolet Trailblazer upside down in Little White Oak Bayou and called police.
A dive team recovered the vehicle and found the man's body inside.
Police say they believe the man left a nearby bar shortly before the crash.
Officials said they are not sure how long the body had been there, but it did not appear to be more than a half day.
The accident remains under investigation. Anyone with information is asked to call police.