Marchand was selected 71st overall in the 2006 NHL Entry Draft by the Boston Bruins. He made his NHL debut on October 21, 2009, against the Nashville Predators.[9] He then scored his first career NHL goal against Jhonas Enroth of the Buffalo Sabres on November 3, 2010.

Marchand established himself as a two-way player for the Bruins during the 2010–11 season, his first full year in the NHL. He scored 21 goals, including five short-handed (ranked third in the league),[10] and 41 points; he also registered a +25 plus-minus rating.[11] On April 2, 2011, prior to the Bruins' final home game of the season, against the Atlanta Thrashers, Marchand was awarded the Bruins' 7th Player Award, as voted by the club's fans through the regional New England Sports Network (NESN). The honour is awarded annually to the Bruins player fans believe performed most beyond expectations. During the 2011 Stanley Cup playoffs, Marchand scored 19 points over 25 games, helping the Bruins to a Stanley Cup championship. His 11 goals tied Jeremy Roenick for the second-most by a rookie in the NHL. Marchand's total included two goals in the seventh and deciding game of the Finals against the Vancouver Canucks.

Marchand during the 2011-12 NHL season.

During the off-season, the Bruins re-signed Marchand to a two-year contract extension, announced on September 14, 2011.[12] During the ensuing season, on December 23, 2011, he scored his first career NHL hat-trick in an 8–0 win against the Florida Panthers at TD Garden. He also added two assists for a five-point effort.[13]

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During the 2014–15 season, Marchand played on the Bruins' top line for the majority of the season, he finished the season as Boston's leading goal-scorer, with 24 goals, and tied for fourth on the team with defenceman Dougie Hamilton in point totals, with 42.[14] Immediately following the 2014–15 regular season, Marchand required surgery on the reportedly torn tendons around his right elbow, something that had been bothering him since the early spring of 2014 — he expected to be fully ready for the 2015–16 Boston Bruins season when training camp began on September 17, 2015.[15]

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During a home game on February 6, 2016 Marchand was given a penalty shot chance against the Buffalo Sabres, a rare occurrence during an overtime tie-breaker round in the regular season — with 2:28 left on the 3-on-3 overtime clock, Marchand scored on the penalty shot, the first time in Bruins team history that a successful penalty shot ever won a regular season game in overtime.[16] Marchand finished the season with 37 goals and 24 assists, and a plus-minus of 21.[17]

Midway through the 2016 World Cup of Hockey, where Marchand was one of the leading scorers with eventual champion Canada, he signed an eight-year contract extension with the Bruins worth $49 million through the 2024–25 season. [18]

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Late in the 2016–17 season, on March 13, 2017, Marchand scored the second hat trick in his Bruins career, with all three goals scored in the third period of a 6–3 road game Bruins' win against the Vancouver Canucks with only his teammate David Krejčí scoring one additional goal in-between Marchand's trio of third-period Bruins' goals.[19]

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Marchand scored his third career hat trick, as well as a pair of assists making for a five-point night, in a 6–5 home-ice overtime victory over the Detroit Red Wings on March 6, 2018.[20] His third goal in this game was the overtime winner, which also made Marchand the Bruins' all-time franchise leader in regular-season overtime goals with the 11th of his career.[21] On March 31, 2019, in a game against the Detroit Red Wings, Marchand scored his 26th career shorthanded goal becoming the Bruins all-time shorthanded goal leader passing Rick Middleton.[22] In the Bruins' next game on April 3, on the road against the Columbus Blue Jackets, Marchand became the tenth Bruins player in team history to score 100 points (36 goals, 64 assists) in a season, the first Bruin to do so since former Bruins center Joe Thornton did in the 2002–03 season.[23]

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As of the end of the 2020–21 regular season, Marchand's 319 goals rank him seventh in Bruins' history.