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.

On July 11, the Eagles narrowly defeated future Duke teammate Harry Giles and Team CP3 74–73 in the Nike Peach Jam Semi-Finals with a game-winning buzzer-beater to advance to the championship game, where Tatum finished the game with 28 points and five rebounds.

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. So yes, they ended up losing in the end.

During the Circuit, Tatum led the EYBL in scoring with 26.5 points to go along with 9.5 rebounds per game. He was one of the best players in the EYBL at the time.

Among his senior year highlights were a 40-point, 17-rebound game in a 76–57 victory over Bentonville High School and its star Malik Monk, a 46-point game against Huntington Prep and Miles Bridges at the Cancer Research Classic, and a final 40-point game against DeMatha Catholic High School and future #1 NBA draft pick Markelle Fultz at the 2016 Hoop Hall Classic.

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. In April, Tatum played in the Jordan Brand Classic, where he scored 18 points in a 131–117 victory over the West team. Tatum was named the 2016 Gatorade National Player of the Year.

In his second year, he averaged 15 points along with a few rebounds, good numbers for a sophomore.

In his third year, Jayson Tatum took a huge leap and became an all-star level player. He led the Boston Celtics to the playoffs, just like in his previous two years. However, a certain person who now plays for the golden state warriors eliminated him and his team in six games. (Former Heat Legend, Jimmy Butler.)

Nothing notable happened in his fourth year.

Tatum also became the first player in Celtic history to win b2b (back-to-back) player of the week honors, and he also won player of the month while averaging 30+ points over two months. He had an incredible regular season and was named to the ALL-NBA FIRST TEAM.

He led the Celtics into the postseason and swept through the first round. In the second round, he was facing the defending NBA champion Milwaukee Bucks, and they gave Boston some problems. In a 7 game series, the Celtics came out on top, thanks to another high-quality performance from Tatum. When going up against the #1 seeded Miami Heat, the Celtics were one three pointer away from losing, and Jimmy Butler got the open look, but he missed it. The Celtics were able to skate past Miami in a 7 game series. Then they went to the NBA Finals, but it was all downhill from there. Tatum averaged a ridiculous 4 turnovers per game, and even finished the playoffs as the all time leader for turnovers in one postseason. However, Boston still took a commanding 2-1 lead in the series, but eventually all of the positive aspects of their game disappeared and the warriors, from down 2 games to 1, went on to win the series by winning three games straight, something nearly unheard of in NBA Finals history. Despite the loss, this was a eye opening and amazing season for Jayson Tatum.

On January 5, 2023, Tatum put up his second career triple-double with 29 points, 14 rebounds, and 10 assists in a 124–95 victory.

This season, Tatum didn't have any old All-Star weekend, he broke multiple records.

On February 19, playing for the NBA All-Star Game, Tatum tallied 55 points, 10 rebounds, and six assists, breaking Anthony Davis's previous All-Star record high of 52 points, and also won the game's Most Valuable Player award. Tatum also became the first player in NBA history to score at least 50 points in the regular season, the playoffs, and the All-Star Game.

In the postseason, the Celtics breezed to the NBA Eastern Conference finals, where they hit their first and final road bump of the season. That would be Jimmy Butler. He had a generational playoff run, and as the 8th seed, as a play-in team, he managed to defeat the Boston Celtics while dragging a below average heat roster with him. Tatum became the first Celtic in Celtic history to average 30+ points through the course of one season.

Then, the time had come. The time for Jayson Tatum's second NBA Finals appearance.

In Game 2 of the 2024 NBA Finals against the Dallas Mavericks on June 9, Tatum recorded a near triple-double with 18 points, 12 assists, and nine rebounds in a 105–98 victory. Three days later in Game 3, he had 31 points, six rebounds, and five assists in 106–99 victory. Tatum and Jaylen Brown became the first Celtics duo to each post at least 30/5/5 in an NBA Finals game. In doing so, they also surpassed Larry Bird and Kevin McHale for the most 25-point games as a Celtics duo in playoff history. Eventually, in a 5 game series, the Boston Celtics won their 18th franchise championship (most by any NBA team, ever) and Jayson Tatum won his first. Controversially, he didn't win Finals MVP, but that doesn't matter.