McKellen and Stewart are longtime friends who co-star as Magneto and Professor Xavier in the "X-Men" film franchise.
"I'm going to marry Patrick," McKellen announced during an interview on last Saturday's edition of the British chat program "The Jonathan Ross Show."
"How else do you put that? I'm going to officiate at his wedding."
McKellen didn't say exactly how he is legally qualified to marry people, but he said he has previously presided over a ceremony "with two guys having a civil partnership. I was crying my eyes out," EW.com said.