League sources told ESPN and the Boston Globe on Friday that Williams is signing a four-year, $54 million extension with the Celtics. The deal will put him among the top half of NBA centers in annual salary.
The team has yet to confirm the agreement.
Since entering the league as a first-round draft pick in 2018 out of Texas A&M, Williams has seen his role increase each season, including 13 starts over 52 games this past season.
Williams averaged career highs in points (8.0), rebounds (6.9), assists (1.8) and blocks (1.8) over 18.9 minutes per game in the 2020-21 campaign.
The 23-year-old Williams, however, has been limited to just 113 games across his first three NBA seasons because of nagging injuries. Despite those injury woes, he has been effective as a finisher and a rim protector.