Sven Linus Ullmark (Swedish pronunciation: [ˈliːnus ˈulmɑːrk]; born 31 July 1993) is a Swedish professional ice hockey player who is a goaltender for the Ottawa Senators of the National Hockey League (NHL).

Ullmark was selected by the Buffalo Sabres in the sixth round, 163rd overall, of the 2012 NHL entry draft and made his NHL debut in 2015. With the Boston Bruins during the 2022–23 season, he led the NHL in wins, goals against average, and save percentage, scored an empty net goal, becoming the 13th NHL goaltender to do so, and won both the Vezina Trophy and William M. Jennings Trophy, the latter being shared with his teammate Jeremy Swayman.

Ullmark began his junior career assigned to Kramfors-Alliansen of the Swedish J18 Elit League in the 2008–09 season, despite remaining part of Modo's organization. However, when one of the goaltenders assigned to Modo's J20 team left, Ullmark rejoined Modo and appeared in one game in the 2009–10 season. He remained with Modo's J20 team for the 2010–11 season, appearing in two games. He split the 2011–12 season between Modo's J20 team and Modo's senior team in the Swedish Hockey League (SHL), playing 25 games in J20 and 3 games in the SHL. He split the 2012–13 season between Modo's J20 team, playing 25 games, registering a 2.04 goals against average (GAA) and a save percentage 0.924, Mora IK in the HockeyAllsvenskan, playing six games with a GAA of 2.10 and a save percentage of 0.904 and six games with Modo's senior team, with a 2.07 GAA and a 0.934 save percentage. He added five games in the playoffs with Modo's senior team. Ullmark played with Modo's senior team only in 2013–14 season, making 35 appearances, with a record of 17 wins and 16 losses (17–16–0), a GAA of 2.08 and a save percentage of 0.931. For his play that season, he was awarded the Honken Trophy as the league's best goaltender. He returned to Modo for the 2014–15 season playing 35 games, with a record of 12–20–0, a GAA of 3.12 and a save percentage of 0.904.

In his first year eligible for the National Hockey League (NHL)'s entry draft in 2011, he was not among those chosen. In the following 2012 NHL entry draft, Ullmark was selected by the Buffalo Sabres in the sixth round, 163rd overall. On 28 May 2014, Ullmark was signed by the Sabres to a three-year, entry-level contract. In his first season in North America, Ullmark attended the Sabres' training camp. After recovery from an early injury, he was assigned to begin the 2015–16 season with the Rochester Americans of the American Hockey League (AHL). Ullmark played in three games before being recalled to the Sabres on 23 October 2015, replacing Nathan Lieuwen, who was sent to Rochester in his place. He made his NHL debut the following day, making 24 saves in a 4–3 defeat to the New Jersey Devils. His first victory came in his second start, a 3–1 win over the Philadelphia Flyers. He appeared in 20 games with Buffalo, with a record of 8–10–2, a GAA of 2.60 and a save percentage of 0.913. In 28 games with Rochester he had a record of 10–16–0, a GAA of 3.41 and a save percentage of 0.902.

Ullmark spent the majority of the subsequent 2016–17 season with the Americans, amassing a record of 26–27–2, a 2.87 GAA and a save percentage of 0.909. He was selected along with teammate Cole Schneider to represent Rochester at the 2017 AHL All-Star Classic. He was recalled multiple times by Buffalo but only saw action at the end of season when he started in the Sabres' final game. The Sabres fell 4–2 to the Tampa Bay Lightning on 9 April 2017, with Ullmark stopping 33 of 37 shots. On 13 June, the Sabres re-signed Ullmark to a two-year, $1.5 million contract. In the 2017–18 season, Ullmark once again spent the majority of the time with Rochester in the AHL, with a record of 21–12–4 in 44 games, a GAA of 2.44 and a save percentage of 0.922. He represented Rochester at the 2018 AHL All-Star Classic alongside teammate C. J. Smith. He was recalled to the Sabres for the first time on 8 January 2018 to replace an injured Robin Lehner. He started his first game of the NHL season on 11 January, earning his only victory in a 3–1 win over the Columbus Blue Jackets. He returned to Rochester after Lehner recovered. He was recalled again in March as the Sabres carried three goaltenders on their, sharing the net with Lehner and Chad Johnson. He appeared in four more games, losing all four of them, before suffering a concussion. He finished his time with Buffalo with a 1–4–0 in five games, a GAA of 2.00 and a save percentage of 0.935. After recovering, Ullmark was sent back to Rochester after they qualified for the 2018 Calder Cup playoffs. However, he fared less well, allowing 16 goals in three games as the Americans were swept by the Syracuse Crunch in their first round, best-of-five series.

Ullmark joined the Sabres full-time during the 2018–19 season after the Sabres let Lehner and Johnson depart as free agents. 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 22 December, Ullmark stopped all 40 shots faced to shut out the Anaheim Ducks 3–0. Overall, in 37 games, he had a 15–14–5 record, with a GAA of 3.11 and a save percentage of 0.905 while sharing the net with Carter Hutton. On 3 August 2019, the Sabres re-signed Ullmark to a one-year, $1.325 million contract. In the following 2019–20 season, again splitting the netminding duties with Hutton, Ullmark went winless in eleven games before suffering an injury to his right leg in a 5–2 loss to the Ottawa Senators on 28 January 2020. He returned to the lineup after missing 18 games on 9 March in a 3–2 shootout win over the Washington Capitals, marking his final appearance before the NHL suspended the season due to the COVID-19 pandemic on 12 March. He finished the season with a record of 17–14–3, a GAA of 2.69 and a save percentage of 0.915.

With the start of the 2020–21 season delayed by the pandemic, Ullmark signed a one-year, $2.6 million contract with the Sabres on 26 October. Ullmark missed a couple of games into the season due to his father passing. He established himself as the Sabres' starting goaltender until suffering a lower-body injury on 25 February 2021. He missed 15 games before returning on 27 March, with Hutton, Dustin Tokarski and Jonas Johansson sharing the net in his absence. The Sabres lost all of the games he missed, plus three more after his return before Ullmark was in net on 31 March for a 6–1 victory over the Philadelphia Flyers to stop their 18-game losing skid, the longest in the NHL since the 2003–04 season. However, his return was short-lived, after suffering another lower-body injury in his eighth game back on 13 April, missing the remainder of the season. He finished the season with a record of 9–6–3 in 20 appearances, a GAA of 2.63 and a save percentage of 0.917.

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 played his first regular season game for the Bruins on 22 October, a 4–1 win against the Sabres. Although rookie Jeremy Swayman started the first couple games of the season, Ullmark started most of the games before the Bruins previous starting goaltender Tuukka Rask's returned from injury in November. However, Ullmark retained his position as starting goaltender with Rask as his backup until February 2022, when Rask retired. He recorded his first shutout with the Bruins his final game of the season on 28 April, a 5–0 win over the Sabres. Ullmark finished the season with a record of 26–10–2 in 41 games, with a GAA of 2.45 and a save percentage of 0.917. Ullmark was named as the starting goaltender for the Bruins for their first round, best-of-seven series against the Carolina Hurricanes in the 2022 Stanley Cup playoffs. Ullmark struggled in the first two games, as he let in four goals in both games, both of which were losses. As a result, the team opted to start Swayman in game three, who started the rest of the series. The Bruins were eliminated by Carolina in game seven.