The man, whose name was not reported, was found about 4 a.m. by a passing motorist, the Houston Chronicle reported.
The motorist told police that when he found the car in a ditch, it was running and the windshield wipers were on. He said he took the keys out of the ignition and opened the trunk to find the body.
He then called police from a nearby phone.
When police arrived, they found the body in the trunk. There was no obvious cause of death, police said.