He began his career in the youth system of Polonia Warsaw. During the 2005/2006 season,Piątek was promoted to the 1st team. Piątek played in 2 league games and 2 cup games during the season. He was substituted in during the 21st minute against Odra Wodzisław Śląski earning his debut. The following season, he played in 16 matches scoring a goal.