Olympiacos Kostoulas started his career with Agia Anna Volou, before joining
Olympiacos in 2019. He progressed through the academy, notably scoring sixteen goals in fifteen appearances in the 2021–22 season, as his side went on to win the under-15 championship. He signed his first professional contract with the club in September 2022. He made his debut for the Olympiacos B team on 24 January 2023, coming on as a substitute for
Giannis Karakoutis in a 4–0
Super League Greece 2 win against
Irodotos, becoming the youngest player in the history of the club to play professionally, beating the record held by
Kyriakos Papadopoulos.
Brighton & Hove Albion On 12 June 2025, Kostoulas signed for
Premier League club
Brighton & Hove Albion on a five-year deal for a fee of €35 million. Later that year, on 25 October, he scored his first goal for the club in a 4–2 away defeat against
Manchester United. ==International career==