Catharism (; from the Ancient Greek: καθαροί, romanized: katharoi, "the pure ones") was a Christian dualist or Gnostic movement between the 12th and 14th centuries which thrived in Southern Europe, particularly in northern Italy and southern France. Followers were described as Cathars and referred to themselves as Good Christians; in modern times, they are mainly remembered for a prolonged period of religious persecution by the Catholic Church, which did not recognize their unorthodox Christianity.

Catharism emerged in Western Europe in the Languedoc region of southern France in the 11th century. Adherents were sometimes referred to as Albigensians, after the French city Albi where the movement first took hold. Catharism was initially taught by ascetic leaders who set few guidelines, leading some Catharist practices and beliefs to vary by region and over time. The movement was greatly influenced by the Bogomils of the First Bulgarian Empire, and may have originated in the Byzantine Empire, namely through adherents of the Paulician movement in Armenia and eastern Anatolia who were resettled in Thrace (Philippopolis).

Among the most notable and controversial beliefs of the Cathars was the idea of two gods or deistic principles, one good and the other evil. The Catholic Church asserted this was antithetical to monotheism, a fundamental principle that there is only one God, who created all things visible and invisible, as stated in the Nicene Creed. Cathars believed that the good God was the God of the New Testament, creator of the spiritual realm, whereas the evil God was the God of the Old Testament, creator of the physical world whom many Cathars identified as Satan. Cathars believed human spirits were the sexless spirits of angels trapped in the material realm of the evil god, destined to be reincarnated until they achieved salvation through the consolamentum, a form of baptism performed when death is imminent, when they would return to the good God as "Perfect".

The Catholic Church denounced Cathar practices, particularly the consolamentum ritual. From the beginning of his reign, Pope Innocent III attempted to end Catharism by sending missionaries and persuading the local authorities to act against them. In 1208, Pierre de Castelnau, Innocent's papal legate, was murdered while returning to Rome after excommunicating Count Raymond VI of Toulouse, who, in his view, was too lenient with the Cathars. Pope Innocent III then abandoned sending Catholic missionaries and jurists, declared Pierre de Castelnau a martyr and launched the Albigensian Crusade in 1209. The nearly twenty-year campaign succeeded in vastly weakening the movement; the Medieval Inquisition that followed ultimately eradicated Catharism by 1350.

There is academic controversy about whether Catharism was a real and organized movement or whether the medieval Church imagined or exaggerated it. The lack of any central organization among Cathars, regional differences in beliefs and practices, as well as the lack of sources from the Cathars themselves has prompted some scholars to question whether Catharism existed. Other scholars say that there is evidence of the existence of Catharism, and also evidence that the threat of it was exaggerated by its persecutors in the Church.

Though the term Cathar () has been used for centuries to identify the movement, whether it identified itself with the name is debated. In Cathar texts, the terms Good Men (Bons Hommes), Good Women (Bonnes Femmes), or Good Christians (Bons Chrétiens) are the common terms of self-identification.