This article shows that the Tribunal in the Chagos Marine Protected Area Arbitration between Mauritius and the United Kingdom has contributed considerably to the creeping expansion of compulsory jurisdiction of courts and tribunals established under Part XV of the United Nations Convention on the Law of the Sea (UNCLOS). The Tribunal has employed three techniques to do so. First, it has read down the jurisdictional precondition to exchange views in Article 283(1) of the UNCLOS; second, it has expanded the limited scope of compulsory subject-matter jurisdiction under section 2 of Part XV by broadening the meaning of the phrase ‘any dispute concerning the interpretation or application of this Convention’ to include incidental, related—and through the backdoor of a balancing exercise—even extraneous disputes; and, third, it has restricted the limitations and exceptions to compulsory jurisdiction in Articles 297 and 298 of the UNCLOS. Few would have predicted in 1982 that a Part XV court or tribunal would—within the context of such a balancing exercise—ever find that a colonial era undertaking created binding legal obligations under international law and that the United Kingdom was obliged to return the Chagos Archipelago to Mauritius when no longer needed for defence purposes. The Tribunal's expansive reading of the jurisdictional provisions in Part XV opens up the possibility of future rulings, albeit incidentally, on issues that have little to do with the law of the sea.