Farewell Horshack

Ron Palillo -- the actor who played Horshack on "Welcome Back, Kotter" -- died earlier this week at his home near Palm Beach, FL from an apparent heart attack ... this according to someone close to the actor.

Palillo was found by his partner of many years Joseph Gramm around 4:00 AM. Gramm called an ambulance and Palillo was rushed to the hospital, where he was pronounced dead.

We're told the heart attack was very unexpected. Palillo was 63.