In the summer of 2015, he joined the St. Louis Eagles Amateur Athletic Union (AAU) team on the highly competitive Nike Elite Youth Basketball League (EYBL) Circuit.

[5] The next day, he had 24 points, seven rebounds, and four blocks in a 104–77 loss to the Georgia Stars and future Duke one-and-done Wendell Carter Jr. in the 2015 Nike Peach Jam championship game.

[7][8] As a senior in 2015–16, Tatum averaged 29.6 points and 9.1 rebounds and posted six 40-point games while leading Chaminade to its second Missouri Class 5A state championship.

[12] Tatum was selected to the 2016 McDonald's All-American Game on March 30, 2016, at the United Center in Chicago, leading the East Team in scoring with 18 points and grabbing eight rebounds in a 114–107 loss.

[34] The next day, he tallied 19 points and eight rebounds in a 75–69 victory over Notre Dame, earning the Blue Devils the ACC tournament championship.

[37] As the #2-seed entering the NCAA tournament, Duke defeated Troy in the first round but exited early in a second-round loss against South Carolina.

[42] During the 2017 NBA Summer League event in Utah, Tatum averaged 18.7 points, 9.7 rebounds, 2.3 steals, and 2.0 assists in nearly 33 minutes of action.

Later, in Las Vegas, Tatum produced similar results, averaging 17.7 points, 8.0 rebounds, 1.0 assists, and 0.8 blocks in nearly 32 minutes of action in three games.

Tatum was named to the All-Summer League Second Team alongside Bryn Forbes, Cheick Diallo, Wayne Selden Jr., and Kyle Kuzma.

[43] In his NBA debut on October 17, 2017, Tatum recorded a double-double with 14 points and 10 rebounds as the team's starting power forward during the season-opening 102–99 loss to the Cleveland Cavaliers.

On April 15, 2018, in Game 1 of the first-round series against the #7-seed Milwaukee Bucks, Tatum recorded a double-double with 19 points and 10 rebounds in a 113–107 overtime victory.

[67] On February 13, Tatum would again score 39 points, while playing 47 minutes and leading the Celtics to a 141–133 double-overtime victory over the Los Angeles Clippers.

[69] As the season was halted by the COVID-19 pandemic, Tatum shot 2-of-18 in a 119–112 loss to the Milwaukee Bucks on July 31,[70] but he bounced back the next game scoring 34 points on 11-of-22 shooting in a 128–124 victory over the Portland Trail Blazers on August 2.

[77] During the narrow season-opening 122–121 victory over the Milwaukee Bucks on December 23, 2020, Tatum totaled 30 points and seven rebounds, alongside a game-winning three-pointer.

[91] On May 28, in Game 3 of the first-round series against the Brooklyn Nets and their "Big Three" (Kevin Durant, James Harden, and Kyrie Irving), Tatum scored 50 points on 50% shooting (16/30), and put up six rebounds, seven assists, and two steals in the 125–119 victory.

[96] On October 25, 2021, in a 140–129 overtime victory over the Charlotte Hornets, Tatum recorded 41 points, seven rebounds, and eight assists to lead the Celtics to their first win of the season.

[111] On May 9, in Game 4 of the Eastern Conference Semifinals, Tatum recorded 30 points, 13 rebounds, and five assists during a 116–108 victory over the reigning champion Milwaukee Bucks to tie the series at 2–2.

[113] On May 15, in Game 7, Tatum posted 23 points, six rebounds, and eight assists in a 109–81 victory, thus securing the Celtics a place in the Eastern Conference Finals.

[114] On May 23, in Game 4 of the Eastern Conference Finals, Tatum recorded 31 points, eight rebounds, and five assists in a 102–82 blowout victory over the Miami Heat to tie the series at 2–2.

Tatum was named the inaugural Eastern Conference Finals MVP after averaging 25.0 points, 8.0 rebounds and 5.5 assists per game in the series.

[128] On January 16, Tatum scored a season-high 51 points, along with nine rebounds and five assists on 15-of-23 shooting, 7-of-12 from three, and 14-of-14 from the free throw line during a 130–118 victory over the Charlotte Hornets.

[130] Three days later against the Golden State Warriors, in a rematch of the 2022 NBA Finals, Tatum recorded 34 points alongside a career-high 19 rebounds, six assists, and three steals during the 121–118 overtime victory.

[139] On March 24, Tatum scored 34 points, setting a franchise record with his 40th 30-point game of the season, and the Celtics rolled to a 120–95 victory over the Indiana Pacers.

On April 27, in Game 6 of the first-round playoff series against the Atlanta Hawks, Tatum recorded 30 points, 14 rebounds, and six assists during the 128–120 close-out victory.

[161] In Game 1 of the Eastern Conference Finals against the Indiana Pacers on May 21, Tatum scored 36 points, including 10 in overtime, along with 12 rebounds, four assists, and three steals in a 133–128 victory.

[170] On November 16, 2024, Tatum put up 24 points, 11 rebounds, and nine assists, alongside a buzzer-beating, game-winning three-pointer in a 126–123 overtime win over the Toronto Raptors.

He became just the fifth player in NBA history to record 40+ points, 15+ rebounds, 10+ assists and 5+ 3s in a game, joining James Harden (2x), DeMarcus Cousins, Russell Westbrook and Vince Carter.

[177] Representing Team USA at the 2016 Nike Hoop Summit, Tatum saw 16 minutes, 57 seconds of action, recording 14 points, four rebounds, two assists, two steals, and a block.

He helped Team USA win the gold medal, averaging 15.2 points, 3.3 rebounds, 1.2 assists, 0.5 steals, and 1.2 blocks while shooting 49.3% from the field in six games.

[222] In February 2024, Tatum partnered with the financial services company SoFi to help people buy homes in his hometown of St.