This study examines the role of the legendary Muslim prophet Khidr in the conversion and contestation of sacred sites in the changing frontier zones of the medieval Islamic world. Khidr, the elusive figure of immortality and esoteric knowledge, played a crucial role in the reformulation of medieval Islamic frontier zones, places where diverse populations came into contact with each other. As a figure who was able to travel great distances in short periods of time, Khidr became known for daily prayer circuits that included prayers at major monuments in Mecca, Jerusalem and, depending on the time period, Cairo, Tunis, and Constantinople. Descriptions of Khidr praying at recently converted sites cast these buildings in a new series of associations with each other to create new sacred geographies. His visits to a number of mosques, tombs, and dervish lodges are recorded in local legends and usually marked by inscription or the prominent display of spolia. After the twelfth century, Khidr's name became appended to the names of many fully converted, partially converted, and Christian sites in the Middle East and Anatolia. At the same time, local Muslim and Christian audiences began to identify Khidr with a number of Christian saints, such as St. George, St. Theodore, St. Behnam, and St. Sergius. Although this identification is well known, there have been few studies that take into account the formal and functional qualities of Khidr sites within these converted buildings and Christian structures. This paper examines Khidr sites in three buildings, the Mar Behnam monastery in Mosul, the Aya Sofya in Istanbul, and the Elwan Celebi Zawiya in Corum to address changes in how Khidr was understood. It argues that as paradigms of converted space changed so did the names and attributes associated with Khidr.