"Sylvester Stallone is devastated and grief-stricken over the sudden loss of his son, Sage Stallone... His loss will be felt forever," said a widely cited statement issued by his publicist, Michelle Bega.
TMZ, which live blogged the breaking news of Stallone's death, cited unidentified sources as saying that he had died of an overdose of pills and that it did not appear to be a suicide.
People magazine reported that officers from the LAPD were called to the house at 2:17 p.m.
The body was initially found by a housekeeper, according to Sage Stallone's attorney George Braunstein, who added that the young actor was not known to be depressed.
"I’m personally devastated. It's just a tragic, tragic loss," Braunstein told NBC TV, adding:
"He was an extremely good actor, a good writer/director, really one of those people who was extremely respectful of Hollywood. He loved his mom and dad. He was a talented, talented person.
"He was thinking of getting married, working on a lot of film projects."
Assistant Chief Coroner Ed Winter told The Associated Press that there were neither signs of foul play nor a suicide note, and that prescription bottles were recovered from the scene.
Sage Stallone was the "Rocky" actor's eldest child with first wife, Sasha Czack.
Sylvester Stallone had been promoting his latest movie, "Expendables 2," when he heard the news, TMZ reported.
"His compassion and thoughts are with Sage’s mother, Sasha," Bega's statement said. "Sage was a very talented and wonderful young man."