This dissertation provides a comprehensive analysis of Riccoldo da Monte Croce's (d. 1320) nascent theology of Islam. Riccoldo's firsthand knowledge of Muslims and his frequent and explicit reference to it in his writings make him a unique figure in medieval interreligious relations. An examination of his theology of Islam as found in his four major works will contribute to the study of historical theologies of religions by investigating a neglected part of that history, the medieval period. It will also enable a comparison between medieval and contemporary theologies of religions. No study in any language has analyzed Riccoldo's theology of Islam as such, nor placed him into conversation with the current theology of religions debate. The dissertation is divided into five chapters. Chapter One discusses the desirability of linking medieval and contemporary Christian views of other religions and defines three key terms: interreligious experience, theological authority, and theology of religions. Chapter Two describes relevant aspects of Riccoldo's historical milieu. Chapter Three outlines those elements of Riccoldo's theology of Islam which are consonant with the mainstream medieval view. Chapter Four focuses on Riccoldo's four major writings, all of different genres, and highlights the unique aspects of his theology of Islam, most notably his use of 'interreligious experience.' Chapter Five compares Riccoldo's nascent theology of Islam to more recent Christian theologies of Islam. It suggests that Riccoldo's views of Islam, while largely consonant with those of his medieval confreres, remain significant, not because he consciously drew upon interreligious experience as a theological authority, but because his honest descriptions of his encounters with Muslims illuminate the nature of interreligious experience as such. By allowing his theology of Islam to retain some of the tension and ambivalence which resulted from his experience of Muslims, Riccoldo demonstrates that medieval Christian views of other religions were far more complex and ambiguous than is often assumed today.