Michel Floréal Chasles (15 November 1793 – 18 December 1880) was a French mathematician.

He was born at Épernon in France and studied at the École Polytechnique in Paris under Siméon Denis Poisson. In the War of the Sixth Coalition he was drafted to fight in the defence of Paris in 1814. After the war, he gave up on a career as an engineer or stockbroker in order to pursue his mathematical studies.

In 1837 he published his Historical view of the origin and development of methods in geometry, a study of the method of reciprocal polars in projective geometry. The work gained him considerable fame and respect and he was appointed Professor at the École Polytechnique in 1841, then he was awarded a chair at the Sorbonne in 1846.

Jakob Steiner had proposed the problem of enumerating the number of conic sections tangent to each of five given conics, and had answered it incorrectly. Chasles developed a theory of characteristics that enabled the correct enumeration of the conics (there are 3264) (enumerative geometry). He established several important theorems (all called Chasles' theorem). That on solid body kinematics was seminal for understanding their motions, and hence to the development of the theories of dynamics of rigid bodies.

In 1865 he was awarded the Copley Medal.

As described in A Treasury of Deception, by Michael Farquhar (Peguin Books, 2005), between 1861 and 1869 Chasles purchased over 27,000 forged letters from Frenchman Denis Vrain-Lucas. Included in this trove were letters from Alexander the Great to Aristotle, from Cleopatra to Julius Caesar, and from Mary Magdalene to a revived Lazarus, all in a fake maedieval French.

Chasles' name is one of 72 that appears on the Eiffel Tower.