Western democracies are experiencing a widespread shift towards greater recognition of transgender rights in public policy, yet the timing of change differs across states. To explain this variation, the author presents a novel theoretical framework called 'policy momentum'. Unlike existing work on policy diffusion, which typically emphasises domestic or international processes, the author theorises how the combined pressure from each level creates the conditions for policy change to occur. Empirically, she contrasts the creation of national human rights policies to protect transgender individuals in Canada (2017) and Australia (2013). Using process-tracing and within-case analyses, and drawing on elite interviews, primary documents, and Hansard records, the author demonstrates the decisive interaction of subnational legislative changes with an emerging global norm to produce transgender policy change. This paper thus contributes to an understanding of LGBTQ+ public policy, while also providing a framework for explaining the conditions for cross-national policy change more broadly.