The Skyfall director, who divorced Kate Winslet in 2010 after seven years of marriage, was said to be involved with Hall at the time of their split, although she later claimed that such rumours were "b*llocks".
A rep for Mendes has now revealed that the pair are together, telling the Daily Mail: "It's true they are in a relationship."
A source close to Mendes added: "They are dating and have been for some time. I can't put an exact date on it, but certainly nothing happened until after Sam's marriage was over."
Winslet recently confessed that she had to remind herself to "have a good cry" following her divorce from Mendes, as she had become too good at "soldiering on".
The Carnage actress is thought to be dating male model Louis Dowler after the pair reunited in June.
Watch the trailer for Rebecca Hall's The Awakening below: