Haarms is the son of Martine van Hoorn and Peter Haarms. He was born and grew up in Amsterdam. He initially played soccer and judo before taking up basketball. Haarms purchased an outdoor hoop at the age of 14 so he could practice more. Haarms joined a club team, the Harlemlakers, and admittedly was very poor at first but soon developed into one of the top talents in the country. After Brord Brugman set up a private practice session, Haarms then moved to Spain and played for Joventut Badalona's junior team. He helped the team reach the championship game of the tournament and hit 5-of-7 three-point shots in the semifinal against Real Madrid. He was offered a professional contract in Spain but declined the invitation. Instead, Haarms moved to the United States to complete a postgraduate year at Sunrise Christian Academy in Bel Aire, Kansas. He started to receive collegiate attention when he joined the Chauncey Billups AAU Elite team. Haarms committed to Purdue over offers from Colorado, Vanderbilt and Washington State, becoming the Boilermakers' sixth international player since 1952. He chose Purdue because he liked coach Matt Painter and the program's record of developing big men.
College career
Purdue
Haarms enrolled at Purdue a semester early and redshirted the second half of the 2016–17 season because the NCAA ruled he would have lost a season of eligibility had he stayed at Sunrise Christian. As a redshirt freshman he averaged 4.8 points, 3.2 rebounds and 2.1 blocks per game in 37 games. His 79 blocked shots were the second-most by a Purdue freshman. Haarms started against Butler and Texas Tech in the NCAA Tournament after Isaac Haas injured his elbow and had a combined 11 points and nine rebounds in the two games. He averaged 9.4 points, 5.4 rebounds, 1.1 assists and 2.1 blocks per game as a redshirt sophomore. On February 19, 2019, Haarms hit a last-second shot to defeat Indiana after being taunted by Indiana fans all game. Haarms suffered a head injury in a loss to Nebraska on December 15, and missed the following game against Ohio. On January 2, 2020, Haarms scored a career-high 26 points on 11-of-16 shooting in a double-overtime win against Minnesota. He suffered a hip injury in a double-overtime loss to Michigan on January 9. As a junior, Haarms averaged 8.6 points, 4.6 rebounds, and 2.0 blocks per game. Following the season, he transferred from Purdue.
On April 23, 2020, Haarms announced that he would transfer to BYU over offers from Kentucky and Texas Tech, gaining immediate eligibility as a graduate transfer.
