On June 26, 2024, Castle was selected with the fourth overall pick by the
San Antonio Spurs in the
2024 NBA draft and on July 2, he signed with the Spurs. He made his
NBA Summer League debut on July 6, putting up 12 points, six rebounds and three assists in a 97–65 loss to the
Charlotte Hornets. Castle was named Western Conference Rookie of the Month for January 2025. On February 7, Castle put up a career-high 33 points in a 117–116 loss to the
Charlotte Hornets. At the beginning of April, Castle was named Western Conference Rookie of the Month for March 2025. On April 29, 2025, he was announced the winner for the
NBA Rookie of the Year award. Castle participated in the 2025 AT&T Slam Dunk Contest but lost in the final round to
Mac McClung. On November 12, 2025, Castle recorded his first career triple-double with 23 points, 10 rebounds, and 10 assists in the Spurs’ 125–120 loss to the
Golden State Warriors. He and
Victor Wembanyama became the first pair of teammates in Spurs history to record triple-doubles in the same game. They also became the fifth duo in NBA history to each produce a 20-point triple-double in a game. On February 7, 2026, Castle put up his second career triple-double with a career-high 40 points, alongside 12 rebounds and 12 assists, on 15-of-19 shooting from the field, in a 138–125 win over the
Dallas Mavericks. With the performance, he became only the second player in Spurs franchise history after
David Robinson to record a 40-point triple-double. On March 12, Castle put up his third career triple-double with 30 points, 11 rebounds, and 10 assists in a 136–131 loss to the
Denver Nuggets. On March 28, Castle recorded his fourth career triple-double with 22 points, 10 rebounds, and 10 assists in a 127–95 win over the
Milwaukee Bucks. On April 24, in Game 3 of the
Western Conference First Round against the
Portland Trail Blazers, Castle put up 33 points and five assists in a 120–108 win. His teammate in
Dylan Harper put up 27 points, as they joined
Kevin Durant and
Russell Westbrook as the only duos to each put up 25 points in the same playoff game at 21 years old or younger in NBA history. ==National team career==