Mats Simon Hjalmarsson (born February 1, 1989) is a Swedish professional ice hockey player. He plays as a forward for Rögle BK in the Elitserien. In 2006–2007, he played for the Västra Frölunda HC in the J20 SuperElit league and Gislaveds SK in the Division I. He was among the very best players in the Swedish U20 league that season, scoring 54 points in 41 games. The St. Louis Blues drafted him 39th overall in the 2007 NHL Entry Draft.