The comedian and actor -- who starred opposite Larry David on his hit HBO show -- passed away Tuesday night ... with Bette Midler being one of the first people who noted it Wednesday. She didn't provide much detail, simply writing, "On a sad note, the great Richard Lewis has died."

According to his rep -- he died from a heart attack at his home in L.A.

It's a shocking turn of events for fans of the beloved 'Curb' star -- as he'd been publicly battling with Parkinson's disease lately ... which he announced he'd been diagnosed with last year.

In his announcement, Richard noted that he'd actually planned on retiring from comedy a few years ago due to a number of health issues -- which all culminated with his Parkinson's diagnosis. He said at the time he was okay and was taking the right medication to manage it.

He appeared relatively okay health-wise despite his Parkinson's diagnosis. The disease hadn't reached a point where it was visibly hindering him or his ability to perform, including his recurring role as his fictional self on 'CYE.' It's unclear if his diagnosis played any part in his passing.

Richard was a regular on the long-running series since the beginning ... playing Larry's best friend, with whom he'd often squabble. Of course, Richard was also a decorated comic.

The last time Richard was seen out in public was actually alongside his wife, Joyce Lapinsky. She appeared to be helping him walk -- as they were out and about in Beverly Hills.

He's survived by his wife. Richard was 76.