Officers responded to the 4400 block of Horseshoe Circle at 5:47 p.m. after receiving reports that a person may be dead inside, police said.
They found the 70-year-old man already dead when they arrived and are investigating his death as a homicide.
Police have not disclosed the man's cause of death nor if there are any suspects in the case.
Anyone with information about the man's death has been asked to call Antioch police at (925) 779-6933.
