At the end of February 2024, alongside Youssoufa Moukoko and Julien Duranville, he was one of three U20 players in the squad for the Champions League round of 16 first leg against PSV Eindhoven.

On matchday 32 of the 2023–24 season, Wätjen made his Bundesliga debut for Borussia in the 5–1 home win against FC Augsburg, where he was in the starting eleven and set up Marco Reus' fourth goal.

[6] He reached the final undefeated with the national team, scoring a stoppage-time goal in a 3–0 victory against Scotland in the group stage,[7] before clinching the title against France by winning on penalties.

[8] Wätjen rejected a call up to Germany's squad for the U17 World Cup in Indonesia in winter 2023 due to upcoming high school exams.

After the fake news regarding his ancestry spread, the Swedish Football Association made immediate efforts to ensure his services.