On 24 June 2019, it was announced that Promes was moving to Eredivisie side AFC Ajax on a five-year deal, with a basic transfer fee of €15.7 million, which could rise to €17.2 million with add-ons. Promes made his debut for Ajax against PSV in the 2019 Johan Cruyff Shield on 27 July 2019, and was replaced in the 59' by Hakim Ziyech in Ajax's 2–0 win.