Kostoulas stayed for a year at his first professional club,
Olympiacos Volos, before moving to
Kalamata for the in 1994. The highlight of his five seasons there, which reaped 135 league appearances, was the club's promotion to the
Alpha Ethniki in 1995. In July 1999, he joined
Olympiacos and ended each of his first three terms with a Greek championship medal. He started the following season on the bench but when handed an opportunity he took it, playing in seven out of the final eight games and excelling in the derby defeat of
Panathinaikos which paved the way for another championship success. He remained a consistent performer when used as the title was lost in 2004 and then won back in each of the next two seasons along with the
Greek Cup. In January 2007, he moved to
Skoda Xanthi and in summer 2011 to
Asteras Tripolis. In the summer of 2013 Kostoulas returned to finish his career in his first professional club, Olympiacos Volos. ==International career==