Kutcher, 35, joined Kunis, 30, in St. Petersburg, Fla., on Saturday to attend her brother’s wedding.
Michael Kunis, 36, tied the knot with ballet dancer Alexandra Blacker, 29, who trained Mila to play a ballerina in 2010’s Black Swan, at the Cathedral Church of Saint Peter. The couple got engaged last December in San Diego.
Following the ceremony, to which Mila wore a blush, knee-length dress, a reception was held at the historic St. Petersburg Yacht Club.
According to a source, Mila and Ashton's present to the newlyweds is their upcoming honeymoon: a 10-day trip to Australia.
Now that Kutcher's divorce from Demi Moore has been finalized, the Two and a Half Men star hopes to walk down the aisle with the Ted actress.
"Ashton and Mila have talked about getting married for quite a while," a business source close to Kutcher tells PEOPLE. Adds another Kutcher pal: "They're incredibly happy together."