Andrew John Casson FRS (born 1943) is a mathematician, studying geometric topology.
Casson is the Philip Schuyler Beebe Professor of Mathematics at Yale University in the United States where he served as department chair between 2004 and 2007. His Ph.D. advisor at the University of Liverpool was C. T. C. Wall, but he never completed his doctorate; instead what would have been his Ph.D. thesis became his fellowship dissertation as a research fellow at Trinity College, Cambridge. He was Professor of Mathematics at the University of Texas at Austin between 1981 and 1986, at the University of California, Berkeley, from 1986 to 2000, and has been at Yale since 2000.
In 1991, he was awarded the Oswald Veblen Prize in Geometry by the American Mathematical Society. In 1998 he was elected to Fellowship of the Royal Society.
Casson 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 to the disproof of the manifold Hauptvermutung, introduced the Casson invariant, a modern invariant for 3-manifolds, and Casson handles, used in Freedman's proof of the 4-dimensional Poincaré conjecture.