Born in Castricum on 28 February 1998 and developing in the youth ranks of AZ Alkmaar, Koopmeiners made his professional debut in the 2017/2018 season, and at just 19 years of age he immediately became one of the Dutch side's key midfielders - also frequently finding himself on the scoresheet thanks to his ball striking abilities, which led to him becoming his side’s primary penalty taker within a few months.

The 2020/2021 campaign really saw him take off, with Koopmeiners scoring 15 goals in 32 games and winning the Eredivisie's Player of the Season award – eventually attracting the attention of Atalanta, for whom the Dutch midfielder has played 129 games over the last three years in all competitions, registering 29 goals and 15 assists.

His numbers, statistics and, above all, performances of the highest level have allowed him to cement himself within the Netherlands national team, whom he represented from Under-17 level upwards before making his senior debut in October 2020, and he has since gone on to make 21 appearances for the senior national side.