After representing the Czech Republic in several youth categories, Souček made his debut for the senior side on 15 November 2016 in a friendly match against Denmark. He made his competitive debut on 10 June 2017, in Czech Republic's 1–1 away draw against Norway in a 2018 World Cup qualifier. Souček appeared in all three matches of the Czech Republic under-21 team in the 2017 UEFA Under-21 Championship.