The Iberian Middle Ages, where for many centuries coexisted Christianity, Islam and Judaism occupies a particular place in the history of religion. This article proposes the research of conversion from Islam to Judaism under the political domination of Christianity. The author uses the sources on the history of the Muslim population of the Crown of Aragon and studies this phenomenon from the point of view of the medieval law, that was reflected in the legal discourse of the Crown Chancery. The cases of Muslims conversion in the Crown of Aragon testify about a neighborhood of Christianity, Islam and Judaism in the social space of the Crown.