Sylvester Stallone's eldest son Sage died from heart failure, the Los Angeles county coroner's office says, adding that there were almost no drugs in the 36-year-old's body.
Sage Moonblood Stallone, who was also an actor and appeared with his father in two films, was found dead at his Hollywood home on July 13, leaving the Rocky star "devastated".
Some prescription pill bottles were taken from the apartment, fueling speculation about the mystery death of a young actor who had shown no signs of depression or ill health.
But the Los Angeles County Coroner's Department listed the cause of death as Atherosclerotic Coronary Artery Disease, adding that he suffered from no other conditions, and describing the death as natural.
"Toxicology was negative except for a sub-therapeutic level of Hydrocodone," a painkiller, added a short statement from the coroner's office, adding that a full report would be published within the next week.
Sylvester Stallone, 66, was told of his son's death while he was at the Comic-Con pop culture festival in San Diego. Reports suggested the young actor may have been dead for a few days before his body was found.
Stallone said there was "no greater pain" than losing a child, and condemned "speculation and questionable reporting" about his son, who was actively planning his own wedding in Las Vegas at the time of his death.
The young actor appeared in the 1990 film Rocky V as Rocky Balboa Jr, the son of his father's title character. The pair also appeared together in the 1996 movie Daylight.
He is credited with 13 big-screen roles as an actor, while he was a producer on three films, including the 2006 short Vic which he also wrote and directed.