Legeno, who starred in the last three instalments of the 'Harry Potter' franchise as werewolf Fenrir Greyback, was found dead in a remote area of Death Valley, California by a pair of hikers.
An official cause of death has yet to be released, but he is thought to have suffered from extreme heat-related issues.
Watson, who played female lead Hermione Granger in the films, took to Twitter to pay tribute to the actor, writing, "I'm so sad to hear about #DaveLegeno. He was such a lovely man. I hope he is resting in peace."