2010-11: Jan Kostalek appeared in one game for the Sparta Prague U20 team – playing most of the season with the Czech club's U18 team – and also played in 19 international games for the Czech U16 team. He had no points nor penalties in his only U20 game. Kostalek scored 2 goals with 9 assists in 39 games with the Sparta Prague U18 team and was plus-four with 34 penalty minutes. Sparta U18 finished first in the regular season but lost in the playoff quarterfinals. Kostalek was plus-one with no points and 8 penalty minutes in five playoff games. In six games for Sparta U16 he scored 1 goal with 3 assists and was plus-five. He scored 3 goals with 6 assists and had 34 penalty minutes playing for the Czech Republic U16 team.
2011-12: Kostalek made his Czech Extraliga debut – playing in 10 games for Sparta Prague – while competing for the club's U20 team and representing the Czech Republic in three international championship tournaments. He had no points and was minus-three with 4 penalty minutes in his pro hockey stint. In 32 games for Sparta Prague U20 he scored 3 goals with 4 assists and was plus-seven with 30 penalty minutes. In the club's four playoff games he was plus-four with no points and 2 penalty minutes. In 10 games for Sparta's U18 team (including three playoff games) he scored 1 goal with 16 assists and was plus-11. He played in the 2011 U19 World Junior A Challenge, 2012 U17 World Hockey Challenge, and 2012 U18 World Junior Championship. Representing the Czech Republic in 15 international matches between the three teams he had 3 assists and 16 penalty minutes. Kostalek was selected by Rimouski in the first round (10th overall) of the 2012 CHL Import Draft.
2012-13: Kostalek impressed as both a physical defenseman and puck mover skating for Rimouski in his first season in North America. Slowed by an injury early in the year, he skated in 48 QMJHL games, finishing with 5 goals and 13 assists, and was plus-17 with 53 penalty minutes. The Oceanic finished second in the East Division. Kostalek had 1 assist and was minus-five with 2 penalty minutes in the six-game playoff series with Gatineau. He finished the year representing the Czech Republic at the 2013 U18 World Junior Championship; playing five games and finishing plus-one with 2 assists. Kostalek played for Team Orr in the CHL/NHL Top Prospects Game and was ranked 43rd amongst North American skaters in Central Scouting's final rankings prior to the 2013 NHL Draft.
An extremely polished two-way defenseman, Kostalek plays a mature game and adapted well to being counted on to play plenty of minutes in all situations. Perhaps his best skill is his intelligence and hockey sense on the ice. Quite simply, he rarely makes a mistake. Only average sized, he skates well and plays with a bit of an edge. Offensively, there is still room to grow, but he makes smart decisions and moves the puck well. He'll continue to improve as he gets stronger.