How does the law determine gender and transgender for the purposes of admission to gender-segregated spaces? This article examines this question to understand how transgender identity is legally constructed in gender-segregated spaces. Using trans feminist legal theory, this article explores how the state conceptualizes and re/incorporates transness in a binary gender order. Through case studies of access to gender transition in gender-segregated educational and carceral spaces, the authors find that judges engage in gender naturalization work to legally construct transgender identity in ways that reinforce sexist conceptualizations of immutable and binary gender. We conclude with a discussion of the implications of this narrow construction of transgender identity for a trans feminist jurisprudence and politics.