The couple reportedly chose the upscale Rosewood Castiglion Del Bosco resort for its nuptials, marrying in a medieval church with views across the Montalcino valley.
A source tells E! News Kate walked down the aisle in a white dress with lace sleeves, and wore a veil, with her hair in loose curls parted on the side. Houston Astros pitcher Justin donned a dark tuxedo and bow tie, while the bridesmaids each wore different dresses in a blush colour.
The romantic occasion capped a huge week for the couple after Justin's Astros won the World Series on Wednesday, defeating the Los Angeles Dodgers in a thrilling seven-game series.
The Other Woman actress and the star pitcher dated on and off for five years. They were spotted holidaying together in St. Thomas in January, 2013, but split later that year before rekindling their romance in early 2014.
Kate announced the couple's engagement in May, 2016 at the Met Gala in New York City, showing off a huge diamond sparkler and telling reporters, "I'm really excited. He asked me right before (baseball) season started so we've been keeping it on the down low for quite a while... I'm excited to finally be able to share it with the world!"
The bride also shared a few details about their big day in a recent interview for Martha Stewart Weddings, revealing they didn't want a huge wedding, and hoped for around 150 guests.
"I just know I want a lot of flowers," she revealed. "And I know I want the petals already down by the time my niece, who will be my flower girl, walks down the aisle."
However, she admitted her floral plans were certain to cause problems for her fiance: "Justin has terrible allergies," she said. "He'll have to take an allergy pill." The weekend nuptials won't come as a complete surprise - Kate shared photos from her bridal shower on 23 October (17) at New York City's famed Plaza Hotel.