Santa Ana police said the man, described as Hispanic and in his 40s, was found lying on his back beside a pickup truck in which he was living about 11 a.m. near Harbor Boulevard and 1st Street.
The Ford pickup truck was parked in an auto shop lot. People in the area said the man often slept in it.
The cause of death was under investigation.
Since Dec. 21, three homeless men have been found dead in separate incidents in the northern side of the county.
A joint task force was created by three law enforcement agencies to investigate the killings.
Police have been handing out fliers warning of the killings and urging the homeless to stay in groups.
The Orange County Rescue Mission in Tustin has been distributing emergency kits and asking transients to find shelter.