The enigmatic Muslim ruler known in Arabic as Muḥammad ibn Sa’d ibn Mardanīsh and in Latin as Rex Lupus fought the Almohads through alliance with Christians. His closest ally was King Alfonso VIII of Castile. Ibn Mardanīsh paid Alfonso tribute and granted him important fortresses, like other Muslim tributaries to Christian rulers. Yet the relationship between these two rulers is more complex than a simple tributary model would suggest. Alfonso had been orphaned and named king at age three in 1158, and his minority was marked by a fierce battle between would-be regents. Ibn Mardanīsh, ruler of Murcia since 1147, appears to have been present frequently at the court of the young king and to have participated in the debates over his regency. Ibn Mardanīsh’s presence is recorded at events like Alfonso’s 1160 donation of a church to a monastery – a transaction between Christians, in which Ibn Mardanīsh had no stake. Even so, a Latin chronicler wrote that Ibn Mardanīsh confirmed this donation, referring to him as a vassal of the king. Chronicles report that Ibn Mardanīsh entered Toledo in 1167, perhaps to adjudicate between the noble families of the Laras and the Castros, who fought each other over the right to the regency. In later years, the Ibn Mardanīsh and Alfonso VIII fought alongside each other against both Christian and Muslim foes, and Alfonso VIII intervened with the king of Aragon to make him stop attacking Ibn Mardanīsh’s territory. Both rulers were criticized for their alliance, and encouraged to focus on fighting the infidel rather than their coreligionists. Yet signs of their close relationship continued even after the death of Ibn Mardanīsh in 1172. Two years later, Alfonso VIII began minting gold coins on the model of his former vassal’s dinars. These coins, inscribed with Christian messages in Arabic, were some of the earliest Christian gold coins minted in Europe, and followed the language and iconography of Ibn Mardanīsh’s coins closely. This paper uses Arabic, Latin and Castilian chronicles and diplomatic treaties alongside material sources to assess the complexities of interreligious vassalage and alliance during a time of great Christian-Muslim conflict. Although chronicles emphasize the enmity between Christians and Muslims, a close examination of the relationship between Ibn Mardanīsh and Alfonso VIII shows that many of the Reconquista’s successes were instead born from Christian-Muslim alliance.