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.
This perceived reliance on meat had been put forward by some as one of the reasons these humans become extinct as large animals such as mammoths declined due to an Ice Age.
But a new analysis of Neanderthal remains from across the world has found direct evidence that contradicts the chemical studies. Researchers found fossilised grains of vegetable material in their teeth and some of it was cooked.