On 15 March 2016, Fournier matched his career high with a 30-point effort against the Denver Nuggets. In 2015–16, Fournier had a career year in which he averaged personal highs in scoring (15.4 ppg), rebounding (2.8 rpg), assists (2.7 apg), steals (1.2 spg) and minutes (32.5 mpg). He led the team in scoring 19 times, had 24 20-point games and two 30-point games. On 7 July 2016, Fournier re-signed with the Magic on a five-year, $85 million contract. On 3 November 2016, he scored a season-high 29 points against the Sacramento Kings. On 16 January 2018, Fournier scored a career-high 28 points plus 4 in a 108–102 win over the Minnesota Timberwolves. On 7 March 2018, against the Los Angeles Lakers, Fournier sprained his left medial collateral ligament. He was subsequently ruled out for "a significant stretch of time". On 1 December 2019, Fournier tied his career-high of 32 points in a game in a 100-96 win over Golden State. Boston Celtics (2021) On 25 March 2021, Fournier was traded to the Boston Celtics in exchange for Jeff Teague and two future second round draft picks. Fournier had played and started in 26 games and was averaging 19.7 points, 2.9 rebounds, 3.7 assists and 1.0 steals, while shooting 38.8 percent from three-point range with the Magic for the season. On 29 March, Fournier debuted for the Celtics against the New Orleans Pelicans but went scoreless in 33 minutes of action. On 2 April, Fournier scored 23 points and logged a career-high seven 3-pointers in a 118–102 win over the Houston Rockets. Fournier put up 20 points of his 23 point-performance in the fourth quarter, becoming the second Celtics player to score 20 points on 100 percent shooting in a quarter since Paul Pierce. He missed several games due to the NBA's COVID-19 health and safety protocols. New York Knicks (2021–present) On 17 August 2021, the New York Knicks acquired Fournier in a sign-and-trade deal with the Celtics. On October 20, Fournier made his Knicks debut, tying a career-high 32 points in a 138–134 double overtime win over his former team, the Boston Celtics. On 6 January 2022, Fournier surpassed that career-high, scoring 41 points behind ten three-pointers, along with eight rebounds, in a 108–105 victory over the Celtics. On 23 March, in a 121–106 win over the Charlotte Hornets, he broke the Knicks' record of most 3-pointers made in a