A converso (Spanish: [komˈberso]; Portuguese: [kõˈvɛɾsu]; Catalan: convers [kumˈbɛrs, komˈvɛɾs]; "a convert", from Latin conversvs, "converted, turned around") and its feminine form conversa was a Jew or Muslim who converted to Catholicism in Spain or Portugal, particularly during the 14th and 15th centuries, or one of their descendents. Mass conversions once took place under significant government pressure. The Treaty of Granada (1491) at the last surrender of Al-Andalus issued clear protections of religious rights; the Alhambra Decree (1492) began the reversal.

New Christians of Moorish origin were known as moriscos (Galician and Portuguese: Mourisco). The term morisco may also refer to Crypto-Muslims, i.e. those who secretly continued to practice Islam. New Christians of Jewish origin were referred to as marranos. The term marrano may also refer to Crypto-Jews, i.e. those who secretly continued to practice Judaism.

Conversos were subject to suspicion and harassment from both the community they were leaving and that which they were joining. Both Christians and Jews called them tornadizo (renegade). James I, Alfonso X and John I passed laws forbidding the use of this epithet. This was part of a larger pattern of royal protection, as laws were promulgated to protect their property, forbid attempts to reconvert them, and regulate the behavior of the conversos themselves, preventing their cohabitation or even dining with Jews, lest they convert back.