Jack Klugman's Cause of Death -- Prostate Cancer

Jack Klugman's cause of death was prostate cancer according to the actor's death certificate ... TMZ has learned.

The document also states Klugman died at his home in the San Fernando Valley. His wife Peggy was by his side.

Klugman -- who died unexpectedly on Christmas Eve -- was most famous for playing Oscar Madison on the legendary TV show "The Odd Couple" ... followed by the title role on "Quincy, M.E." in the 70s and 80s.

The actor had already beat cancer once in the 80s -- and although the throat cancer battle cost him his voice, he eventually trained himself to speak again.