The body of the younger Stallone, also an actor and producer-director, was found by his housekeeper, said his lawyer, George Braunstein.
"We don't have any of the details," Braunstein said. "He wasn't depressed. He was thinking about getting married. He had his whole life ahead of him."
The elder Stallone was said to be "devastated and grief-stricken" by his son's death.
Born May 5, 1976, in Los Angeles, Sage Moonblood Stallone was the first son of Stallone and actress Sasha Czack. He began his acting career in "Rocky V," the 1990 installment of the "Rocky" movie franchise. As a 14-year-old, he played Robert Balboa Jr., son of his father's Rocky Balboa character.
Young Stallone again appeared with his father in "Daylight" in 1996 and had roles in nine other movies and short films. His most recent appearance was in a 2011 television documentary on the "Rocky" films.
In addition to acting, Stallone was a co-founder with film editor Bob Murawski of Grindhouse Releasing, which specializes in the theatrical and video release of restored B movies from the 1970s and '80s. The company's catalog includes "An American Hippie in Israel," "I Drink Your Blood" and "Cannibal Holocaust." Its latest release was 2010's "Gone With the Pope."
"Sage was a very talented and wonderful young man," said Sylvester Stallone's publicist, Michelle Bega. "His loss will be felt forever."
Sage Stallone, who had no children, was briefly married to Starlin Wright. That union was annulled in 2008.
Times staff writers Geoff Boucher and John Horn contributed to this report.