The free-agent inked a four-year, $128 million contract, according to the Boston Globe.
"This is a big day in Celtics history," said Celtics president of basketball operations Danny Ainge. "Adding Gordon as a current All-Star, in the prime of his career, to an already good group of players is reason for excitement."
A first-time NBA All-Star in 2016-17, the 6-foot-8 Hayward logged career highs with 21.9 points and 5.4 rebounds, while also adding 3.5 assists, 1.00 steal and 34.5 minutes in 73 games (all starts) with the Utah Jazz last season.
"Whether you're talking about the team's past, present, or future, the Boston Celtics are truly special," Hayward said. "I can't wait to be part of the winning culture of Boston, and to join my teammates and coach Stevens as we work together to bring Celtics fans Banner 18."
Hayward was one of 10 NBA players to produce at least 20.0 points, 5.0 rebounds and 3.0 assists while also shooting 47.0 percent or better from the field last season.
"He's a tremendous competitor and a great teammate who constantly seeks improvement, and consistently impacts winning," Celtics coach Brad Stevens said of Hayward, who played his college ball for Stevens at Butler. "I am looking forward to working with him, once again, as we all continue to strive to compete for championships here in Boston."
Hayward capped his All-Star season by averaging a team-best 24.1 points, 6.1 rebounds, and 3.4 assists 11 playoff games for the Jazz. He scored 20 or more points in eight of those 11 contests as Utah advanced to the Western Conference semifinals for the first time since 2009-10.
Originally selected with the ninth overall pick of the 2010 draft by Utah, Hayward, 27, has posted career averages of 15.7 points, 4.2 rebounds, 3.4 assists, 1.02 steals and 31.3 minutes in 516 games played over seven seasons with the Jazz.
"Gordon brings a great work ethic and character along with his ability to shoot, pass, and create offense in the pick and roll," Ainge said. "His athleticism, size and versatility will allow him to fit in perfectly defensively as well."