This doctoral thesis analyses how and to what effects international law has been involved in making gender a personal legal identity characteristic that is assigned to individuals at birth. It examines various fields of public international law, including norms on development, humanitarian law, human rights law, and passport regulations. It further studies rules of private international law related to legal gender recognition as well as domestic legal situations. The thesis concludes that attributing a legal gender to individuals has fostered state-building processes in several ways and facilitated the codification of hetero-patriarchy and cisnormativity in the law. Looking at recent trends in international law, the author argues that gender (identity) is increasingly treated as a private property that is owned by individuals and legitimately creates capital. This reflects a liberal rights logic, ignoring that legal gender categories are never valuable in themselves, but that they become valuable through surrounding social and legal relations. The author thus contemplates ways to “de-propertize” legal gender by lowering their importance for the accumulation of economic, symbolic, and political capital. Assemblage thinking is thereby used to understand and illustrate how legal gender categories become valuable through the entanglement of norms from different normative orders and their interrelationship with other hierarchisation processes, such as racialisation and classicism. It serves as a queer-feminist method that allows putting at the centre of the analysis feminist concerns about difference and universalism as well as the idea of law as a system of relations.