Andrew John Casson FRS (born 1943) is a mathematician, studying geometric topology.

Casson is the Philip Schuyler Beebe Professor of Mathematics[1] at Yale University.

Casson was educated at Latymer Upper School and Trinity College, Cambridge, where he graduated with a BA in the Mathematical Tripos in 1965.

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 Michael Freedman's proof of the 4-dimensional Poincaré conjecture.

In 1991, he was awarded the Oswald Veblen Prize in Geometry by the American Mathematical Society.