The body was found in a white pickup truck parked on the 27000 block of Avenida Armijo in Laguna Niguel.
Orange County Sheriff's deputies call it a suspicious death.
Homicide detectives were called to the scene.
Neighbors say the man who was found had been renting a residence in the neighborhood for the past few months. The truck, a Dodge Ram, was parked across from the residence.
The incident was reported just before 2 p.m. Monday.
Authorities say the victim is a 34-year-old man. The truck reportedly belongs to the victim.
A witness was walking by and saw the victim's feet hanging out.
At least one neighbor heard the victim arguing with another man shortly before the man's body was found.
"Deputies responded, they found a deceased man approximately 34 years of age in the front seat," said Orange County Sheriff's Spokesman Jim Amormino. "Coroner investigators are currently examining the body, so we do not have a motive or a cause of death at this time. Witnesses did describe a person leaving the scene. That person matching that description was located about a mile and a half away from here. He's currently being questioned by investigators. A person of interest at this time."
The victim was not identified pending notification of next of kin.