ESPN's Chris Forsberg tweeted a photo of Irving in his new jersey:
Throughout his six-year tenure with the Cavs, Irving wore No. 2. He has history with the No. 11, though, as he wore it at St. Patrick High School in Elizabeth, New Jersey.
By trading for Irving, the Celtics added a four-time All-Star, a former NBA champion and a player who is coming off his most productive NBA campaign, as he averaged a career-high 25.2 points per game last season.
ESPN's Brian Windhorst reported in July that Irving asked for a trade in an effort to emerge from LeBron James' shadow.
He will have that opportunity in Boston while still playing alongside some All-Star-caliber players, such as Gordon Hayward and Al Horford.
The Celtics gave up a ton to land Irving, including an All-Star guard, a strong defensive forward, a promising big-man prospect and a pick that could be near the top of the 2018 draft.
Boston now has just four holdovers on its roster from a team that earned the No. 1 seed in the Eastern Conference and reached the Eastern Conference Finals last season.
The Celtics and Cavaliers still appear to be the conference's best teams, which means Irving may have the opportunity to show what he can do against LeBron on a big stage in 2017-18.