Prince Constantijn of the Netherlands (Constantijn Christof Frederik Aschwin; born 11 October 1969) is the third and youngest son of the former Dutch queen, Beatrix, and her husband, Claus von Amsberg, and is the younger brother of the current Dutch monarch, King Willem-Alexander of the Netherlands. He is a member of the Dutch Royal House and currently fourth in the line of succession to the Dutch throne.

Prince Constantijn was born in Utrecht, following the births of his brothers, Willem-Alexander (born in 1967), and Johan Friso (1968–2013). He goes by the nickname Tijn. His godparents are former King Constantine II of Greece, Prince Aschwin of Lippe-Biesterfeld, Axel Freiherr von dem Bussche-Streithorst, Max Kohnstamm, and Corinne de Beaufort-Sickinghe.

Prince Constantijn studied Law at Leiden University, becoming a lawyer, and then worked at the Brussels department of (Dutch) European Union commissioner of foreign relations Van den Broek. Later, he was hired by the EU, and continued to work there in various capacities until the end of 1999. In December 2000, he was awarded a Master of Business Administration at INSEAD in Fontainebleau, France. He then spent a summer working for the International Finance Corporation of the World Bank group in Washington, D.C.. He worked until late 2002 for strategic consultants Booz Allen Hamilton in London. Since 2003 he works for the RAND Corporation Europe in Brussels. Furthermore, he has a part-time position at the Netherlands Ministry of Foreign Affairs in The Hague.

Prince Constantijn rarely attends public events in his capacity as a member of the Dutch Royal House. Prince Constantijn is a keen sportsman and enjoys football, tennis, golf and skiing. His other hobbies include drawing, cooking and reading.

The engagement of Prince Constantijn and Petra Laurentien Brinkhorst was announced on 16 December 2000. The civil marriage was conducted by the mayor of The Hague, Wim Deetman, in the Oude Raadzaal, Javastraat, The Hague, on 17 May 2001. The church wedding took place two days later on 19 May in the Grote of St Jacobskerk, with Reverend Carel ter Linden officiating.

Prince Constantijn and Princess Laurentien have three children:

Countess Eloise Beatrix Sophie Laurence, Jonkvrouwe van Amsberg (born 8 June 2002 in Bronovo Hospital in The Hague)
Count Claus-Casimir Bernhard Marius Max, Jonkheer van Amsberg (born 21 March 2004 at Bronovo Hospital in The Hague)
Countess Leonore Marie Irene Enrica, Jonkvrouwe van Amsberg (born 3 June 2006 at Bronovo hospital in The Hague)
The family recently moved from Brussels to The Hague.

Upon the abdication of Queen Beatrix on 30 April 2013, the children of Prince Constantijn and Princess Laurentien ceased to be members of the Royal House, although they continue to be members of the royal family and remain in the line of succession