Hayward initially revealed the news in an article for the Players' Tribune on July 4.
According to ESPN.com's Chris Forsberg, Hayward agreed to a max contract of four years and $128 million.
The finalization of the deal was delayed by Boston's need to clear enough salary cap space to make it happen.
Happy 37th Birthday Dwyane Wade
Steph Is a Few Shots Away from NBA 3-Point History
Can Harden Keep His Dominance Going?
Steph Gifts Fan Who Asked for Girls UA Kicks with New Curry 6s
Happy 34th Birthday to LeBron 👑
4 Years Ago, Kobe Passed Jordan on the NBA Scoring List
Drummond and Embiid Reignite Rivalry
Happy 24th Birthday to Giannis Antetokounmpo
D-Rose Turned Back the Clock and Put Up 50
Dubs Trolled Fergie So Hard It Became a Challenge
CP3-Rondo Blowup Was a Long Time Coming
NBA Let Players Know They Have to Cover Branded Tattoos
The NBA Is Back and the Soccer World Is Pumped
Boban Is Back to Break It Down for Another Season
Players Battle Campers in Rivalry of the Summer
Happy 30th to KD!
Andrew Bynum Is Making an NBA Comeback
Kobe's Hottest Kicks 👟
The Kyrie-I.T. Trade Shook the NBA 1 Year Ago Today
Dyckman Courts Are the Red Carpet of Streetball
Boston renounced the rights of multiple players, including center Kelly Olynyk, before trading guard Avery Bradley to the Detroit Pistons for forward Marcus Morris.
The movement was enough to make room for Hayward, who will reunited with his college head coach at Butler, Brad Stevens.
Boston finished first in the Eastern Conference during the 2016-17 regular season, and it has added an All-Star to the mix who has gotten better with each passing year.
The 27-year-old enjoyed a career year last season with 21.9 points, 5.4 rebounds and 3.5 assists per game. He also helped the Utah Jazz advance to the second round of the playoffs.
Hayward will join a deep and talented roster in Boston that already includes dynamic offensive guard Isaiah Thomas, All-Star big man Al Horford and rookie No. 3 overall draft pick Jayson Tatum.