Jan Arnold Albert Ketelaar (21 April 1908, Amsterdam 23 November 2001, Lochem) was a Dutch chemist and author of the textbook Chemical Constitution: an Introduction to the Theory of the Chemical Bond (). Van ArkelâÂÂKetelaar triangles are named after him.
Ketelaar's doctoral students included Sol Kimel and Robert de Levie. He was elected a member of the Royal Netherlands Academy of Arts and Sciences in 1958.