Researchers in the US have found grains of cooked plant material in the teeth of the remains.
The study is the first to confirm that the Neanderthal diet was not confined to meat and was more sophisticated than previously thought.
The research has been published in the Proceedings of the National Academy of Sciences.
The popular image of Neanderthals as great meat eaters is one that has up until now been backed by some circumstantial evidence. Chemical analysis of their bones suggested they ate little or no vegetables.