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.

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.