This article proposes the use of transgender theory within medieval studies as both a productive and a politically significant optic. The article employs transgender theory to effect a new reading of the miraculous transformation of the character of Blanchandin/e, in the fourteenth-century French chanson de geste, Tristan de Nanteuil, from female to male. First, the often-overlooked importance of Judith Butler’s analysis of sex and gender for the understanding of transgender and non-normatively-gendered identities is addressed. Next, using theoretical work by Deleuze, and by Deleuze and Guattari, the article demonstrates how the rhizomatic and folding structures that a transgender reading of Blanchandin/e’s transformation brings to light cohere with the series of rhizomes and folds which structure the genealogical logic of the text as a whole. The family tree of Tristan de Nanteuil is shown to answer to queer, rhizomatic, and folding imperatives. In this way, the article demonstrates that the text’s transgender genealogy contradicts the anti-generative model of queerness proposed by queer theory’s antisocial turn.