The past decades have seen the development of externalisation policies, with border controls and the administration of asylum systems increasingly taking place outside a State’s territory. One form of externalisation is extraterritorial processing, whereby States transfer asylum-seekers to another State to process their asylum claim and/or provide international protection. Yet the growing number of extraterritorial processing agreements stands in stark contrast with concerns about their legality, as they face challenges before multiple bodies, including domestic judiciaries as well as regional and international accountability mechanisms. Accordingly, this article explores the ability of multiple legal regimes to constrain the proliferation of extraterritorial processing agreements. It does so by focusing on three externalisation policies: Australia’s offshore processing in Papua New Guinea and Nauru, the asylum partnership between the United Kingdom and Rwanda, and the Protocol between Italy and Albania. The analysis focuses on the question of whether the policies are legal under domestic, regional and international legal regimes and to what extent litigation under multiple legal regimes can challenge these extraterritorial processing arrangements. The final section reflects on the added value of the coexistence of multiple legal regimes in the light of the proliferation of such externalisation policies.