Forest Whitaker's ex-wife, Keisha Nash Whitaker, died from alcoholic liver disease ... TMZ has learned.

We obtained Keisha's certificate of death -- issued by L.A. County -- and the document says Keisha suffered from the disease for years before it ultimately killed her.

The certificate also says acute renal failure -- a symptom of anorexia -- was another significant factor contributing to her death.

TMZ broke the story ... Keisha died last month, and the death certificate shows she was at Northridge Hospital Medical Center in L.A. County when she passed.

Keisha was married to Forest for more than 20 years before he filed for divorce in 2018 ... and they had 3 daughters together.

She worked in Hollywood as a model, and her death certificate makes note of her career ... saying she worked 15 years in the modeling industry.

Keisha met Forest on the set of the 1994 film, "Blown Away," and the couple tied the knot years later ... in 1996.

She was only 51 at the time of her death.