The former actor, who hasn't been on camera for over a decade, passed away Sunday at his home in San Bernardino County -- where his body was discovered in his bedroom ... this according to the coroner's office there. The exact circumstances of his death are unclear at this point ... but officials tell us that, for now, there appears to be no foul play involved.

Evan's father, Michael, was able to give us a bit more detail -- telling TMZ Evan was found at a sober-living home. We're told he'd struggled with drugs in the past, but was doing better of late -- and that his sudden passing comes as a complete shock to the family.

Ellingson is perhaps best remembered starring opposite Cameron Diaz -- playing her teenage son -- in 2009's "My Sister's Keeper." He was also well-known for his recurring role as Kyle Harmon on "CSI: Miami" ... in which he starred for a total of 18 episodes over 3 years.

He'd actually gotten his start in showbiz at the age of 13 with a small role in a TV movie as well as a guest appearance on 'General Hospital.' From there, EE went on to act in shows like 'Titus,' 'That Was Then,' 'Mad TV,' 'Complete Savages,' 'Bones,' '24' and others.

Ellingson also acted in a handful of films, such as 'Walk the Talk,' 'Letters from Iwo Jima,' 'The Bondage,' 'Confession,' 'Rules of the Game,' 'Time Changer,' 'The Gristle' and more.

Like we said, Ellingson appears to have stepped away from Hollywood ... not having any acting credits post-2010. Still, he left his mark on TV/film -- and he died quite young.

Ellingson was only 35.