The two married at the Brazilian Court Hotel in Palm Beach, Florida, where 250 guests -- including RHONY star Dorinda Medley (who was a bridesmaid), Jill Zarin, Marysol Patton, Cynthia Bailey and Kelly Bensimon -- celebrated their nuptials with dinner by Daniel Boulud before ringing in the New Year.
De Lesseps wore a custom Randi Rahm gown. She and D'Agostino shared their first dance to "Just Like Heaven" by The Cure.
A wedding guest tells ET that "the wedding was such a blast! Everyone had a wonderful time. Luann and Tom looked so happy. They really got to have fun with all their guests. It was a New Years Eve to remember! You can really tell how much they love each other. Their friends couldn't be happier for them!"
This is the second marriage for de Lesseps, who got engaged to D'Agostino in February. She was previously married to entrepreneur Count Alexandre de Lesseps from 1993 to 2009, with whom she shares two children.
De Lesseps couldn't help but share her excitement on social media in the days leading up to the wedding.
"Soon going from Mrs. DeLesseps to Mrs. D'Agostino Thank you @marysolpatton for the cute purse xo," she captioned one snap of a clutch reading "Mrs D" on Friday.
ET sat down with the 51-year-old reality star in an exclusive interview at her home in the Hamptons in August, where she revealed just how excited she was to tie the knot, even after she discovered that D'Agostino had kissed another woman at a hotel bar just days before proposing.
"We love each other," she shared. "I always say if you can't forgive the people you love, then who can you forgive? … I'm not going to let a silly kiss ruin the rest of our lives."
"I think it was a silly move and something that he feels really terrible about, and that meant really nothing to him," she said, adding that she was looking forward to her nuptials on New Year's Eve, which also happened to be D'Agostino's 50th birthday.
In April, de Lesseps told ET that she was going above and beyond in the dress department for her big day.
"You know what, it's threefold. It's Tom's birthday, he's going to be 50, it's our wedding, and it's New Year's Eve, so I feel like I should change it up, so I'm going to do three different dresses," she revealed. " Why not? For my big night, why not have three different dresses?"