Catharism (from Greek: καθαρός, katharos, pure) was a name given to a Christian religious movement with dualistic and gnostic elements that appeared in the Languedoc region of France and other parts of Europe in the 11th century and flourished in the 12th and 13th centuries. The movement was extinguished in the early decades of the thirteenth century by the Albigensian Crusade, when the Cathars were persecuted and massacred and the Inquisition was set up to finish the job.

Catharism had its roots in the Paulician movement in Armenia and the Bogomils of Bulgaria which took influences from the Paulicians. Though the term "Cathar" has been used for centuries to identify the movement, whether the movement identified itself with this name is debatable. In Cathar texts, the terms "Good Men" (Bons Hommes) or "Good Christians" are the common terms of self-identification.