The prophetic lady in the early fifteenth-century romance Thomas of Erceldoune has received little attention in discussions surrounding this romance text. This essay discusses the affinities this lady shares with the sibyls of ancient Greece and Rome, as well as later manifestations of the sibyls in medieval theology, prophecy, and romance. By drawing upon imagery associated with the prophetic narratives nationhood circulating in the British Isles, the eschatological prophecies of the sibyls Erythraea, Tiburtine, and Cumae, and the many romance Sebiles, the poet validates the Scottish prophecies contained in the text. Allusions to the medieval sibylline tradition also place the narrative and prophecies in this romance within the wider prophetic tradition of medieval Europe. Erceldoune’s lady is a composite character who is heavily dependent upon sibylline allusions. Through these allusions, the lady becomes conduit through which the importance and validity of the Scottish prophecies are communicated in this narrative.