On March 25, 2012, Krug was signed to an entry-level contract by the Boston Bruins, making his NHL debut on April 3, 2012, against the Pittsburgh Penguins.
Appearing in two games with the Bruins, Krug finished with one assist and had an even plus-minus rating, averaging just over 17 minutes of ice time per game.

Krug spent the 2012–13 season with the Providence Bruins, where he was hampered by a sprained ankle until mid-January.
After being called up from Providence on an emergency basis, he scored his first NHL goal during his first Stanley Cup playoff game in the Eastern Conference Semi-finals against the New York Rangers, becoming the first Bruins defenseman to score in his playoff debut since Glen Wesley in 1988.
Krug followed this by scoring his second NHL goal during Game 2 of the same series, becoming the fourth player in Bruins history to score goals in his first two playoff games.
At the conclusion of the series, he became the first rookie defenseman in League history to score four goals in his first five playoff games.
In the 2013 postseason, Krug impressed by scoring four goals and recording six assists.

Krug started the 2013–14 season on the Bruins' opening night roster, and scored his first regular season NHL goal on October 5, 2013, against Jimmy Howard of the Detroit Red Wings.
On November 25, playing against the Pittsburgh Penguins, he became the first rookie defenseman in Bruins history to score an overtime goal, which also proved to be his first NHL game-winning goal.
Krug was frequently mentioned during the 2013–14 season as a strong contender for the Calder Memorial Trophy, awarded to the NHL's rookie of the year.
He was named to the NHL's All Rookie Team and was the top scoring rookie defenseman in the league and was 4th in the league in the goals amongst rookies.
His 14 goals and 26 assists helped the Bruins win the Presidents' Trophy, with the league's best record.
Krug ended the 2014 Stanley Cup playoffs with 10 points in 12 games.

In the 2014–15 Boston Bruins season, Krug tallied 12 goals and 27 assists and recorded a +28 plus-minus, the best of his career.
The following season, he produced 44 points on four goals and a career-high 40 assists.

On June 30, 2016, he re-signed a four-year deal worth an AAV of $5.25M with the Bruins.
Krug ended the 2016–17 season ranking second among the team in assists and sixth in total points.
In late April, Krug underwent surgery to repair a right shoulder injury and was expected to need six months to recover.
Krug recovered enough to participate in the 2017 preseason but was injured in his second exhibition game, forcing him to miss the Bruins' season opener.

While playing in the 2018 Stanley Cup playoffs, Krug injured his left ankle during Game 4 against the Tampa Bay Lightning and was later ruled out for the rest of the series.
By the time of the pre-season schedule, just before the 2018-19 started on October 3 with a road game against the 2018 Stanley Cup-winning Washington Capitals, Krug suffered a different injury to his left ankle in the last 2018–19 pre-season game against the Philadelphia Flyers, and was expected to be out for a three-week period for recovery and re-evaluation.
On March 27, 2019, in a game against the New York Rangers, Krug became the all-time points leader by an American-born player in Boston Bruins history.
On June 1, 2019, during game 3 of the 2019 Stanley Cup Finals against the St. Louis Blues, Krug became the first player in Bruins history to record four points in a Stanley Cup Final game.
On March 25, 2012, Krug was signed to an entry-level contract by the Boston Bruins, making his NHL debut on April 3, 2012, against the Pittsburgh Penguins. Appearing in two games with the Bruins, Krug finished with one assist and had an even plus-minus rating, averaging just over 17 minutes of ice time per game.
Krug spent the 2012–13 season with the Providence Bruins, where he was hampered by a sprained ankle until mid-January. After being called up from Providence on an emergency basis, he scored his first NHL goal during his first Stanley Cup playoff game in the Eastern Conference Semi-finals against the New York Rangers, becoming the first Bruins defenseman to score in his playoff debut since Glen Wesley in 1988. Krug followed this by scoring his second NHL goal during Game 2 of the same series, becoming the fourth player in Bruins history to score goals in his first two playoff games. At the conclusion of the series, he became the first rookie defenseman in League history to score four goals in his first five playoff games. In the 2013 postseason, Krug impressed by scoring four goals and recording six assists.
Krug started the 2013–14 season on the Bruins' opening night roster, and scored his first regular season NHL goal on October 5, 2013, against Jimmy Howard of the Detroit Red Wings. On November 25, playing against the Pittsburgh Penguins, he became the first rookie defenseman in Bruins history to score an overtime goal, which also proved to be his first NHL game-winning goal. Krug was frequently mentioned during the 2013–14 season as a strong contender for the Calder Memorial Trophy, awarded to the NHL's rookie of the year. He was named to the NHL's All Rookie Team and was the top scoring rookie defenseman in the league and was 4th in the league in the goals amongst rookies. His 14 goals and 26 assists helped the Bruins win the Presidents' Trophy, with the league's best record. Krug ended the 2014 Stanley Cup playoffs with 10 points in 12 games.
In the 2014–15 Boston Bruins season, Krug tallied 12 goals and 27 assists and recorded a +28 plus-minus, the best of his career. The following season, he produced 44 points on four goals and a career-high 40 assists.
On June 30, 2016, he re-signed a four-year deal worth an AAV of $5.25M with the Bruins. Krug ended the 2016–17 season ranking second among the team in assists and sixth in total points. In late April, Krug underwent surgery to repair a right shoulder injury and was expected to need six months to recover. Krug recovered enough to participate in the 2017 preseason but was injured in his second exhibition game, forcing him to miss the Bruins' season opener.
While playing in the 2018 Stanley Cup playoffs, Krug injured his left ankle during Game 4 against the Tampa Bay Lightning and was later ruled out for the rest of the series. By the time of the pre-season schedule, just before the 2018-19 started on October 3 with a road game against the 2018 Stanley Cup-winning Washington Capitals, Krug suffered a different injury to his left ankle in the last 2018–19 pre-season game against the Philadelphia Flyers, and was expected to be out for a three-week period for recovery and re-evaluation. On March 27, 2019, in a game against the New York Rangers, Krug became the all-time points leader by an American-born player in Boston Bruins history. On June 1, 2019, during game 3 of the 2019 Stanley Cup Finals against the St. Louis Blues, Krug became the first player in Bruins history to record four points in a Stanley Cup Final game.
Torey Krug