'Saved By The Bell' Dennis Haskins Quietly Battled Parkinson's Disease Before Death

"Saved By The Bell" star Dennis Haskins was battling Parkinson's disease for years before his death ... and he tried to keep it all a secret, TMZ has learned.

Dennis' ex-wife, Sarah Quinones, tells TMZ ... the actor -- who famously played Principal Richard Belding on "Saved by the Bell" -- stayed private about his health struggles in recent years, cutting off communication with his closest confidantes.

We're told Dennis ended up in an assisted living facility -- and multiple sources tell us he was suffering from Parkinson's disease. His official cause and manner of death have not been disclosed.

Quinones tells us ... Dennis didn't want anyone to know that he was suffering from his illnesses, so he wouldn't return phone calls and text messages from Quinones and his friends, who were asking him how he was doing.

On Monday night, Dennis' agent Jay Schachter announced the actor's death, telling TMZ ... "It's a tragic loss. Everyone loved Mr. Belding and those of us who got to know Dennis, loved him even more. He was a great person, and he loved, I mean absolutely LOVED his fans. He always had time for them, no matter what."