Thijs debuted at the international stage during the 2002 European Aquatics Championships in Berlin. Van Valkengoed's main achievement is winning the bronze medal in the 200 m breaststroke at the European Short Course Swimming Championships 2003 in Dublin, Ireland. Earlier that month he had qualified himself for the 2004 Summer Olympics with national records in 100 m and 200 m breaststroke. At the Olympics he was eliminated in the semi finals of the 100 m breaststroke, and in the heats of the 200 m breaststroke.