Mertens made her debut for the Belgium national team on 10 June 2021, coming on as a substitute for
Chloë Vande Velde against
Spain. At the start of 2022, Mertens helped Belgium win the
Pinatar Cup in Spain for the first time, beating
Russia on penalties in the final after a 0-0 draw. Mertens went on to contribute to Belgium's successful qualification for
UEFA Women's Euro 2025 via the play-offs, and was selected in the squad for both legs of the play-off final against
Ukraine. On 11 June 2025, Mertens was called up to the Belgium squad for the
UEFA Women's Euro 2025. ==Honours==