The deal, which is fully guaranteed, comes with the expectation that Pritchard will have a more significant role off the Celtics' bench this season.
"It feels really good," Pritchard said after Boston's first preseason game Sunday night. "Just to be secure, lock it in and be a part of a great organization, a great city. Obviously, I think this is the best basketball organization and city in the NBA. It feels good."
Pritchard is the seventh player in the 2020 draft class to agree on a rookie-scale contract extension. The others are Minnesota's Anthony Edwards, Charlotte's LaMelo Ball, Indiana's Tyrese Haliburton, Detroit's Isaiah Stewart, Memphis' Desmond Bane and San Antonio's Devin Vassell.
The deadline for signing an extension is Oct. 23.
Pritchard, 25, played a career-low 13.4 minutes per game last season, but those minutes are expected to elevate again this year. The 26th pick in 2020 out of Oregon, Pritchard averaged 19.2 minutes and 14.1 minutes, respectively, in his first two NBA seasons.
Pritchard has shot 42.6% on catch-and-shoot 3-pointers since entering the NBA in 2020, per Second Spectrum. That ranks him in the league's top 15 among players with 300-plus attempts in that span. In the final game of the 2022-23 regular season, Pritchard made and assisted on 17 3-pointers -- tied for most by any player ever in an NBA game, according to the Elias Sports Bureau.
Also, he joined James Harden as the only players with a triple-double and nine 3-pointers in a game in NBA history, according to ESPN Stats & Information. Pritchard had 30 points, 14 rebounds and 11 assists in the final game of last regular season.
ESPN's Tim Bontemps contributed to this report.