Étienne Ghys (born 29 December 1954) is a French mathematician. His research focuses mainly on geometry and dynamical systems, though his mathematical interests are broad. He also expresses much interest in the historical development of mathematical ideas, especially the contributions of Henri Poincaré. Alumnus of the École normale supérieure de Saint-Cloud, Ghys obtained his Ph.D. in 1979 from the University of Lille with thesis "Sur les actions localement libres du groupe affine" written under the direction of Gilbert Hector.