[6] Koumas appeared in the senior squad for the first time on 14 December 2023, remaining an unused substitute in a 2–1 loss to Union Saint-Gilloise in the Europa League.
[7] He was later an unused substitute in the EFL Cup final on 25 February 2024, in which Liverpool beat Chelsea 1–0 after extra time at Wembley Stadium.
[8] Koumas made his first team debut for Liverpool in the FA Cup fifth round against Southampton on 28 February 2024, scoring his first senior goal for the club late in the first half in an eventual 3–0 win.
[9] On 10 August 2024, Koumas signed a new long-term contract with Liverpool before joining Championship side Stoke City on loan for the 2024–25 season.
[16] Koumas made his debut for the Wales under-21 team on 22 March 2024, scoring the second goal in the 2–1 win in 2025 UEFA European Under-21 Championship qualifying against Lithuania.