'Diff'rent Strokes' Star Melanie Watson Dead at 57

Melanie Watson Bernhardt -- an actress known for her role on the hit TV show "Diff'rent Strokes" -- has died ... TMZ has learned.

Robert Watson -- Melanie's brother -- tells us ... Melanie passed away Friday in Colorado Springs.

We're told Melanie -- who was born with osteogenesis imperfecta, a disease where a person's bones fracture easily -- had been in the hospital after suffering some issues with bleeding, where she quickly deteriorated.

Robert says the doctors did their best to help her ... but, in reality, she was fortunate to live as long as she did with her condition. he adds he will always think of his sister a great deal.

Fans of classic sitcoms will remember Melanie as Kathy Gordon on "Diff'rent Strokes" ... a part she played in 4 of the show's 189 episodes.

A 1982 episode was memorably named after her character ... in which she and Gary Coleman's character, Arnold, argued when he tried to convince her to walk without crutches.

Bernhardt retired from acting after "Diff'rent Strokes" ended ... later marrying a man named Roger Bernhardt from 1994 to 1996. She reportedly helped found the charity Train Rite to help properly train dogs to work as service animals.

Melanie was 57.