Boston Celtics (2015–2017)
On February 19, 2015, Thomas was traded to the Boston Celtics in exchange for Marcus Thornton and a 2016 first-round pick. During his Celtics tenure, Thomas emerged as a star, a fan favorite, and an "elite crunch-time scorer".
Thomas made his debut for the Celtics on February 18, 2015, in a 118–111 overtime loss to the Los Angeles Lakers, scoring 21 points off the bench. On March 2, 2015, he was named Eastern Conference Player of the Week for games played Monday, February 23, through Sunday, March 1. Thomas later missed eight games in March with a bruised lower back. On April 8, he scored a season-high 34 points in a 113–103 victory over the Detroit Pistons. Thomas subsequently earned Eastern Conference Player of the Week honors for games played Monday, April 6, through Sunday, April 12. In his first career playoff game on April 19, Thomas recorded 22 points and 10 assists in a first-round Game 1 loss to the Cleveland Cavaliers. The Celtics went on to lose to the Cavaliers in a four-game sweep, with Thomas averaging 17.5 points and 7.0 assists during the series. As a result of his sixth-man role with both Phoenix and Boston in 2014–15, Thomas finished second in the NBA Sixth Man of the Year Award voting.
Thomas continued to play for the Celtics during the 2015–16 season. On December 16, 2015, Thomas tied his then-career high of 38 points in a 119–116 loss to the Detroit Pistons. On January 28, 2016, Thomas was named an Eastern Conference All-Star reserve for the 2016 NBA All-Star Game. He became the lowest draft pick to be named an All-Star since the NBA draft was reduced to two rounds in 1989. Thomas also became only the ninth player under 6 feet (1.8 m) to be named an All-Star, while also tying Calvin Murphy as the shortest player to be selected for the All-Star Game. On February 8, 2016, Thomas was named Eastern Conference Player of the Week for games played Monday, February 1, through Sunday, February 7.
With the Celtics down 2–0 to the Atlanta Hawks in the first round of the 2016 playoffs, Thomas scored a then-career-high 42 points in Game 3 in Boston to lift the Celtics to a 111–103 victory. He became only the ninth Celtic to score 40 points in a playoff game. With 28 points in Game 4, Thomas helped the Celtics even the series at 2–2 with a 104–95 overtime victory. However, the Celtics went on to lose the series in six games.
Thomas scored 20 or more points in 20 of the first 21 games of the 2016–2017 season. He was injured in a December 5, 2016, game against the Houston Rockets and received a platelet-rich plasma injection. After returning from a four-game absence for what the Celtics described as a groin strain, Thomas continued to put up big numbers. On December 20, he scored a then-career-high 44 points in a 112–109 overtime victory over the Memphis Grizzlies. He also matched his career best for three-pointers, going 7-of-10 from outside the arc. Thomas earned Eastern Conference Player of the Week honors for the week spanning December 19 through December 25. On December 30 against the Miami Heat, he scored 29 of his career-high 52 points in the fourth quarter, setting a franchise record for points in a period and leading Boston to a 117–114 victory. Thomas' 52 points was the fourth-highest scoring total in Celtics history—Larry Bird holds the record, having scored 60 points in March 1985 game. On January 3, 2017, Thomas recorded 29 points and a career-high 15 assists in a 115–104 victory over the Utah Jazz.
On January 26, 2017, Thomas was named an Eastern Conference All-Star reserve for the 2017 NBA All-Star Game. He averaged 32.9 points in January, marking the third-highest monthly average in team history. On February 2, Thomas was named Eastern Conference Player of the Month for January. He led the NBA in scoring (32.9 ppg) and tied for fifth in the East in assists (6.9 apg) for the month as the Celtics went 10–4 to take over first place in the Atlantic Division. On February 15, Thomas scored 33 points in a 116–108 win over the Philadelphia 76ers. It was his 40th straight 20-point game, tying a team record set by John Havlicek during the 1971–72 season. The following day, Thomas broke Havlicek's team record with a 41st straight 20-point game; he scored 29 points in a 104–103 loss to the Chicago Bulls. With 19 points against the Atlanta Hawks on February 27, Thomas' franchise-record streak of consecutive 20-point games ended at 43. It was only the second time during the season that he did not score at least 20 points in a game.
Thomas suffered an injury in a March 15, 2017, game against the Minnesota Timberwolves and missed the following two games. With 32 points against the Milwaukee Bucks on March 29, he became only the sixth Celtic ever to score 2,000 points in a single season. Thomas also extended his streak with at least one three-pointer to a franchise-best 50 straight games. Behind his leadership, the Celtics finished the 2016–2017 season as the top seed in the Eastern Conference; the team's record was 53–29. Thomas finished the regular season as the third-leading scorer in the NBA, averaging 28.9 points per game.
In the 2017 playoffs, Thomas led the Celtics to a first-round series victory over the Chicago Bulls after learning the day before the series began that his younger sister, Chyna, had died in a car accident. This was the Celtics first playoff series win in five years. After the Celtics defeated the Bulls in six games, Thomas flew cross-country to attend his sister's funeral. Thomas returned to Boston for Game 1 of the Eastern Conference Semifinals and helped the Celtics defeat the Washington Wizards 123–111. According to ESPN, during Game 1, "Thomas lost a front tooth following an inadvertent elbow from Otto Porter, requiring surgery to replace it and fix two other shifted teeth. He played through the dental wreck and scored 33 points and notched nine assists in the win. He didn't miss a game". Two days later in Game 2, Thomas scored 53 points—the second-highest total in Celtics playoff history—to help Boston win 129–119 in overtime and take a 2–0 lead in the series. He became only the fifth Celtic to score 50 or more points in a postseason game, missing John Havlicek's team record by one point. In Game 7, Thomas scored 29 points and had 12 assists to help the Celtics advance to the Eastern Conference Finals for the first time since 2012 with a 115–105 victory.
After the Celtics lost the first two games of the 2017 Eastern Conference Finals to the Cleveland Cavaliers, Thomas was ruled out for the rest of the postseason with a hip injury. In a statement, the Celtics said: Isaiah Thomas will miss the remainder of this year's postseason following re-aggravation of a right femoral-acetabular impingement with labral tear during Game 2 of the Eastern Conference Finals against Cleveland. Thomas initially injured the hip during the third quarter of the Celtics' March 15 game against Minnesota... the injury was further aggravated during Game 6 of the Eastern Conference Semifinals at Washington on May 12.
The Celtics were defeated by the Cavaliers in five games in the Eastern Conference Finals.
Thomas finished fifth in the NBA's most valuable player (MVP) voting for the 2016–17 season.
