William Elgaard Kaastrup (born 21 March 2004) is a Danish footballer who plays as a left-back for Swedish Allsvenskan club Västerås, on loan from Danish Superliga club Randers FC.

On 21 January 2023, Randers FC confirmed that the club had signed 18-year old Kaastrup from F.C. Copenhagen, effective from the summer of 2023.^[2] Five days later, however, Randers confirmed that the clubs had agreed on a deal that sent Kaastrup to Randers with immediate effect instead.^[3] Kaastrup signed a deal until June 2026.^{[citation needed]}

After a few appearances for Randers' U19 team, Kaastrup made his official debut for his new club on 23 April 2023 against Brøndby IF, replacing Adam Andersson for the last few minutes.^[4] In pursuit of more playing time, on January 31, 2024, Kaastrup was loaned to Swedish Allsvenskan club Västerås until the end of the year.^[5]