Known in English as Wenefred, Gwenfrewy († ca. 650) was the primary female saint of medieval Wales. According to her legend, she was beheaded for refusing the sexual advances of a prince but was resurrected by her uncle, St. Beuno, to live a second life as a nun. She died again years later as abbess of Gwytherin, a monastery thirty miles from Holywell, the site of her martyrdom in North Wales. In 1138 Gwenfrewy was translated to Shrewsbury Abbey in England, from which time she was officially venerated in two countries. Previous studies have examined her manifestation in Welsh or English culture specifically, or have evaluated a restricted number of texts dealing with her legend. By contrast, this dissertation is a chronological survey of all available literary evidence for the development of both her Welsh and English cults over the course of the Middle Ages. Through close reading of this evidence and by situating it in its political and historical contexts, this study explores the division between the saint's cult sites in Wales, the impact of oral tradition on her earliest Latin vitae, and the extent to which nationalist interests influenced the spread of her cult and legend in later medieval England. Although she was occasionally claimed as an English saint in the fifteenth century, Gwenfrewy remained for most people living in the medieval British Isles a distinctly Welsh figure, albeit one who could be employed to court Welsh acceptance of English political hegemony. Still, textual evidence from late medieval Wales reveals a seemingly nationalist response to Gwenfrewy's continued appropriation by the English-- the attempt to counter the cultural loss of her relics by overlooking her 1138 translation. One original contribution of this study is the examination of the composite Latin Life of Wenefred in BL Lansdowne MS 436, a text that reveals the currency of Gwenfrewy's legend in England in the fourteenth century. Additionally, Osbern Bokenham's verse Lyf of Seynt Wenefrede from the Abbotsford MS is here fully transcribed and analyzed for the first time, and it reveals the widespread popularity and importance of Gwenfrewy's cult in fifteenth-century England.