With the Boston Bruins during the 2022–23 season, Ullmark led the NHL in wins, goals against average, and save percentage, scored an empty net goal, and won both the Vezina Trophy and William M. Jennings Trophy, the latter being shared with his teammate Jeremy Swayman.

In his season debut on 13 October, he recorded his first career NHL shutout, stopping all 36 shots faced in a 3–0 win over the Arizona Coyotes.

On 28 July 2021, Ullmark left the Sabres as a free agent and signed a four-year, $20 million contract to be the starting goaltender with the Boston Bruins.

Ullmark was named as the starting goaltender for the Bruins for their first round series against the Carolina Hurricanes in the 2022 Stanley Cup playoffs.

[11][12] Ullmark was named one of three Boston representatives to the 2023 NHL All–Star Weekend, joining head coach Jim Montgomery and teammate David Pastrňák.

[13] On 25 February, he became the thirteenth NHL goaltender to score a goal, and the first since Pekka Rinne in 2020, as he shot the puck directly into the Vancouver Canucks' empty net with less than one minute remaining in the Bruins' 3–1 win.

Ullmark and Swayman together received the William M. Jennings Trophy as the goaltending tandem to allow the fewest goals against.

Ullmark finished off the season on a strong note, recording a 1.90 GAA and .935 save percentage in his final nine games after the deadline.

[21] On 24 June 2024, Ullmark was traded to the Ottawa Senators in exchange for Mark Kastelic, Joonas Korpisalo, and a 2024 first-round pick.