The appearance of the "Maggie May" singer, 74, at their wedding on Wednesday -- along with a guitarist and a violinist -- topped off a roller-coaster couple of weeks for the pair.
Andrew Aitchison and Sharon Cook, from Liverpool, England, nearly had to cancel their wedding when their travel operator, Thomas Cook, collapsed on September 23.
However, their wedding venue, Caesars Palace, heard of their dilemma on the news and offered to help make the wedding happen, with Delta organizing flights for them and their wedding guests.
Although the happy couple had been warned by Caesars Palace to expect surprises at the wedding, they were astonished when Stewart appeared at the altar and serenaded them.
Aitchison posted his thanks the following morning on his Twitter account, describing the day as a "dream."
He wrote: "We woke up today with quite a hangover not sure this actually happened, it was like a dream. You helped make it the best day of our lives. Thank you from the bottom of our hearts, you are an absolute Gent and legend Sir Rod. My only worry is that Sharon didn't want to let you go!"
Stewart is currently in Las Vegas for a set of shows at The Colosseum at Caesars Palace. The newlyweds will be seeing him perform live for the second time when they go to his concert on Friday.