Having represented the Netherlands at various youth levels, Koopmeiners was called up to the senior team's preliminary UEFA Nations League squad in August 2020.

Koopmeiners grew up in Castricum, North Holland, and took his first steps in football as a youth player for the local club, Vitesse '22.

[5] He eventually progressed through the ranks, and as part of the club's reserve team, Jong AZ, he became champion of the Dutch third division in the 2016–17 season, reaching promotion to the Eerste Divisie.

[6] On 18 August 2017, Koopmeiners made his professional debut for Jong AZ in a 3–1 away win over FC Den Bosch.

[7] He made his first team-debut on 1 October, coming on as a substitute for Alireza Jahanbakhsh for the final 30 minutes of a 4–0 home loss to Feyenoord.

[13] On 14 January 2021, Koopmeiners scored two goals, including an audacious backheel flick, to help his side clinch an 3–1 away victory against PSV Eindhoven.

[24] He made his competitive debut in a 2022 FIFA World Cup qualification against Turkey on 7 September 2021, contributing with an assist in a 6–1 victory.