In the 2015–16 season, Dončić became a regular member of Real Madrid's senior team. He played against the Boston Celtics in an NBA preseason game on October 8, 2015, collecting 4 rebounds, 1 assist, and 1 block. On October 16, at age 16, he debuted in the EuroLeague, scoring 2 points in a loss to Khimki. Dončić became the 21st player ever to debut in the EuroLeague before turning 17. On October 18, he recorded 10 points and 4 rebounds in a 94–61 win over Gipuzkoa. On November 29, Dončić posted a season-high 15 points, 6 rebounds, and 4 assists against Bilbao. With the performance, he set a new ACB record for most points and highest Performance Index Rating (PIR) in a game for players under age 17. He scored 12 points and grabbed 5 rebounds on January 7, 2016, against CSKA Moscow in the EuroLeague. After a Real Madrid time-out in the second quarter of the game, Dončić made three consecutive three-pointers, recording 9 points in a 2-minute span. Through 39 games in the 2015–16 ACB season, he averaged 4. 5 points, 2. 6 rebounds, and 1. 7 assists. In 12 EuroLeague games, he recorded 3. 5 points, 2. 3 rebounds, and 2 assists per game.