Marranos were Spanish and Portuguese Jews living in the Iberian Peninsula who converted or were forced to convert to Christianity during the Middle Ages, but continued to practice Judaism in secrecy.

The term specifically refers to the charge of crypto-Judaism, whereas the term converso was used for the wider population of Jewish converts to Catholicism, whether or not they secretly still practised Jewish rites. Converts from either Judaism or Islam were referred to by the broader term of "New Christians."

The term marrano came into later use in 1492 with the Castilian Alhambra Decree, which prohibited the practice of Judaism in Spain and required all remaining Jews to convert or leave, under the premise that, "If they are not good Christians, their descendants will be."

By then, the vast majority of Jews in Spain had converted to Catholicism, perhaps under pressure from the Massacre of 1391, and conversos numbered hundreds of thousands. They were monitored by the Spanish Inquisition and subject to suspicions by Catholics of the secret practice of Judaism, also known as "Marranism."

In modern use "marrano" may be considered offensive; and "crypto-Jew" is occasionally preferred in scholarly works.