Kostoulas started his career with Agia Anna Volos, before joining Olympiacos in 2019.
[1][2] 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.
[3] He signed his first professional contract with the club in September 2022.
[4] 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.
[6] Olympiacos U-19 Hailing from a footballing family, Kostoulas is the son of former Greek international footballer Athanasios Kostoulas, while his brother, Konstantinos, plays for Rio Ave on loan from Olympiacos.