Andrew John Casson FRS (born 1943) is a British mathematician, an expert on geometric topology, and a member of the Department of Mathematics at Yale University in the United States. He served as department chair from 2004 to 2007.
Casson's Ph.D. advisor at the University of Liverpool was C. T. C. Wall, but he never completed his Ph.D. What would have been his Ph.D. thesis became his fellowship dissertation as a research fellow at Trinity College, Cambridge, though he has been the advisor to over 20 doctoral students. He was Professor of Mathematics at the University of Texas at Austin from 1981 to 1986, at the University of California, Berkeley from 1986 to 1999, and has been at Yale since 2000.
Casson's has worked in both high-dimensional manifold topology and 3 and 4 dimensional topology, using both geometric and algebraic techniques. Among other discoveries, he contributed significantly to the disproof of the manifold Hauptvermutung, introduced the Casson invariant, an important modern invariant for 3-manifolds, and Casson handles, used in Freedman's proof of the 4-dimensional Poincaré conjecture.