Former Disney star Lee Thompson Young died from a self-inflicted gunshot wound to the head ... this according to the actor's official death certificate obtained by TMZ.
We broke the story ... Young's body was discovered at his North Hollywood apartment on August 19th, after he failed to show up to the set of his TNT drama "Rizzoli and Isles."
According to the death certificate, Young used a handgun to take his own life.
Young -- who rocketed to stardom back in 1998 as the title character in the Disney show "The Famous Jett Jackson" -- didn't leave a suicide note.
Young's friends and family are still struggling to understand why the actor shot himself -- though some of his family members told law enforcement he suffered from "depression issues."
Young was 29.