In this article, I examine how conflicts over transgender bathroom rights have ignited debates concerning the fundamental nature of transgender identity. Through an institutional and discursive analysis of North Carolina's House Bill 2 or “bathroom bill,” the Title IX case in Gloucester County School Board v. G. G., and similar federal court cases, I explore how and why forces both on the right and in the LGBTQ movement have come to rely on scientific expertise to legitimate their conceptions. As conservatives have marshaled evidence to challenge notions that transgender identity is innate, LGBTQ and transgender organizations as well as the American Civil Liberties Union have crafted a “born this way” biopolitical construction of transgender identity. I find that at their core, these conflicts are over the meanings of gender and sex in relation to transgender identity. Conservatives posit sex as biologically rooted and gender as a psychological phenomenon, whereas transgender advocates subsume gender identity into the definition of sex in arguing that constitutional and federal civil rights law must recognize gender identity as a biologically constitutive element of sex. I conclude by noting the limits of a liberal assimilationist and litigation-centric transgender politics and by exploring alternatives to this biopolitical form of transgender political identity.