Sheriff's officials said the call came in at 11:30 p.m. Monday from a home on Mayapan Road.
Emergency crews arrived to find the boy, who did not live at the home, unresponsive.
The child, described as younger than 10 years old, was rushed to an area hospital, where he was pronounced dead.
Investigators are calling it an accidental drowning.
The number of drownings are climbing fast this year. Orange County has had 13 and Riverside has had seven. The majority occurred in backyard pools.