The 36-year-old was an aspiring filmmaker who also acted in two movies with his father in the 1990s, including his debut as the son of boxer Rocky Balboa in Rocky V.
His body was found in his Los Angeles flat by a housekeeper after friends became concerned that they had not heard from him for some time.
There were no signs of foul play, police said. The district’s assistant chief coroner Ed Winter, said there was no suicide note but prescription medicine bottles had been recovered from the home.
An autopsy will be performed in the next few days but it may take several weeks to complete toxicology tests that will establish the cause of death. “Sylvester Stallone is devastated and grief-stricken over the sudden loss of his son, Sage Stallone,” said a representative for the actor.
“His compassion and thoughts are with Sage’s mother, Sasha. Sage was a very talented and wonderful young man. His loss will be felt forever.”