A former Denmark youth international, Hämäläinen made his breakthrough as part of Lyngby Boldklub before moving to Belgian Pro League club Zulte Waregem in 2011.

On 20 August 2007, he made his debut in a 1–6 loss to Esbjerg fB, coming on as a substitute for Andreas Bjelland in the 82nd minute.

[8] In the following season, Hämäläinen played in two games in the Belgian Cup and was eliminated with Genk in the quarter-finals, while in the UEFA Europa League the club reached the last sixteen.

[7] In the 2016–17 season, Hämäläinen won his second Belgian Cup as part of Zulte Waregem, beating Oostende in the final.

[12][13] In January 2021, he suffered a knee injury in practice; tests later confirmed that he had sustained ligament damage which would sideline him for an extended period of time.

[16][17] After the club had won promotion back to the Superliga, Hämäläinen signed a one-year contract extension, keeping him in Lyngby until 2023.