The Dallas Mavericks and restricted free agent Grant Williams are finalizing a four-year, $53 million contract as part of a three-team sign-and-trade agreement with the Boston Celtics and San Antonio Spurs, sources told ESPN’s Adrian Wojnarowski on Wednesday.

San Antonio will receive forward Reggie Bullock and an unprotected first-round pick swap in 2030 from Dallas, while both the Celtics — Williams’ former team — and the Mavericks will receive two second-round picks, sources said.

The Celtics also will get a $6.2 million trade exception, per ESPN’s Bobby Marks.

Acquiring Williams, a burly 24-year-old reserve who averaged 8.1 points and 4.6 rebounds for the Celtics this past season while shooting 39.5% from 3-point range on 3.7 attempts per game, gives Dallas a young, versatile 3-and-D forward who should be a perfect complement to the Mavericks’ star-studded backcourt of Luka Doncic and Kyrie Irving.

By acquiring Williams in a sign-and-trade, Dallas still has access to its full midlevel exception, giving it an option to add another player in free agency. Sources have told ESPN’s Tim MacMahon that Dallas has interest in Portland Trail Blazers restricted free agent forward Matisse Thybulle.

Williams, the 22nd pick in the 2019 NBA draft out of the University of Tennessee, has been a key part of Boston’s runs to the NBA Finals and Eastern Conference finals in each of the past two seasons. While he vacillated in and out of coach Joe Mazzulla’s rotation at times this season, he returned to a big role off the bench in the playoffs. He also did so a year ago under Ime Udoka as Boston advanced to the NBA Finals for the first time since 2010.

After taking on significantly more salary this offseason by acquiring Kristaps Porzingis from the Washington Wizards in a deal that saw franchise stalwart Marcus Smart wind up with the Memphis Grizzlies, Boston opted to get two second-round picks rather than match a deal to retain Williams next season, when the Celtics should once again be one of the top contenders in the East.

The 6-foot-6 Williams, who has an ability to knock down open 3-point shots, was one of the more intriguing restricted free agents on the market, particularly for contending teams. He is a career 43% shooter on corner 3-pointers, compared to 34% on all other 3-point attempts, according to research from ESPN’s Stats & Information. He also is a versatile defensive player.

The Mavericks hope he will fulfill a similar role, as Dallas seeks to bounce back from last season’s disappointing 11th-place finish in the West, which saw the Mavs fail to qualify for the play-in tournament.

Williams had surgery on his left (non-shooting) hand after the season, but the Celtics said he was expected to be ready for training camp.

It has been a busy offseason for the Mavericks, who re-signed Irving to a three-year deal, made two draft-night trades to wind up with a pair of first-round picks — center Dereck Lively II and forward Olivier Maxence-Prosper — and signed guard Seth Curry.

San Antonio, meanwhile, gets to the salary floor by acquiring Bullock, a career 38% 3-point shooter, as well as landing yet another potentially significant future draft asset in the first-round swap.