After many years as a successful player in Poland’s youth divisions, Kostrzewski started his professional career for the division 3 team OSSM PZKosz Sopot where he gained experience and played for Gdynia’s junior team as well. In 2008 he had his first games for Gdynia’s senior team and played his first Euroleague game in 2010. Between 2008 and 2010, he gained further experience at Gdynia’s 2nd and 3rd team as well as its youth teams.
2008: Polish U20 Championships All-Tournament 1st Team
2007: Poland national under-18 basketball team
2008, 2009: Poland national under-20 basketball team