Tom Ilmanen (1961–2025) was an American mathematician specializing in differential geometry and the calculus of variations. He was a professor at ETH Zurich. He obtained his PhD in 1991 at the University of California, Berkeley with Lawrence Craig Evans as supervisor. Ilmanen and Gerhard Huisken used inverse mean curvature flow to prove the Riemannian Penrose conjecture, which is the fifteenth problem in Yau's list of open problems, and was resolved at the same time in greater generality by Hubert Bray using alternative methods.