Jan Camiel Willems (18 September 1939 – 31 August 2013) was a Belgian mathematical system theorist who has done most of his scientific work while residing in the Netherlands and the United States.
On 1 February 1973 he was appointed professor of systems and control in the mathematics department of the University of Groningen.
He served terms as chairperson of the European Union Control Association and of the Dutch Mathematical Society (Wiskundig Genootschap).
In the 1980s Willems worked on the geometric theory of linear systems, where he introduced the notion of almost invariant subspace.
Since the 1990s, he has devoted his interest to the development of the behavioral approach to systems theory and control.