Kaj Sierhuis (born 27 April 1998) is a Dutch professional footballer who plays as a forward for Eredivisie club Fortuna Sittard.

[3][4] He had started playing football with WSV '30 in Wormer in the province of North Holland as a five-year-old before moving the to Ajax Youth Academy at age 11.

[7] Sierhuis made his first competitive appearance for the Ajax first team on 25 July 2018 in a 2–0 win in the UEFA Champions League second qualifying round against Sturm Graz.

[13][14] He made his debut for the club on 5 February 2020, starting in a league game against Nice, but had to be replaced by Nathanaël Mbuku in the 13th minute after falling out with a hamstring injury.

[17] In the 2020–21 season, he made regular appearances for Reims, but his playing time decreased from the beginning of 2021 and he was sent on a one-season loan to Eredivisie club Heracles Almelo in August 2021.