Lisa Robin Kelly -- who played Eric Forman's older sister on "That '70s Show" -- has died ... TMZ has learned.

The 43-year-old actress passed away in her sleep Wednesday night at a rehab facility in California.

We spoke with Kelly's agent who tells us the actress had just voluntarily checked herself in for treatment for an alcohol problem this past week ... and was determined to clean herself up.

The agent tells us, "She had been fighting demons for a while and finally lost her battle."

More Here Via TMZ