The body was discovered about 9:30 p.m. Sunday behind a business at 2028 Hancock St., the San Diego Police Department said in a statement.
Someone walking in the area came across the body and called authorities.
"The victim presented no visual signs of trauma," the statement said. "However, the scene and the condition of the body are considered suspicious."
The victim's identity was not immediately determined. An autopsy was scheduled to be performed Tuesday afternoon, according to the police statement.
Anyone with information on the man's death was asked to call the SDPD homicide unit at (619) 531-2293 or San Diego Crime Stoppers at (888) 580-8477.