Stewart and the singer-songwriter were married Sunday after dating for five years.
"I did my part," McKellen wrote on his official Facebook page, alongside a photo of himself holding up a black T-shirt that read "Minister" and the couple's marriage certificate.
"Yes, married," Stewart tweeted, without offering any further details about the wedding.
UsMagazine.com said the newlyweds are on their honeymoon, but the celebrity news website didn't report where their holiday is taking place.
McKellen and Stewart are known for playing super-powered foes in the X-Men film franchise. They are to co-star in the two-drama repertoire "No Man's Land" and "Waiting for Godot" on Broadway, starting next month.