The United Kingdom’s withdrawal from the 1964 Fisheries Convention and from the European Union has raised questions concerning the potential existence of third states’ fishing rights in the UK’s maritime zones post-Brexit. Historic fishing rights are a complex and controversial issue in the law of the sea. Uncertainty remains regarding their nature, the process for their formation and ascertainment, as well as their contemporary relevance in the light of the Law of the Sea Convention (LOSC) and other fisheries agreements. This article explores the concept of historic fishing rights in the law of the sea. First, it examines the nature and scope of historic fishing rights as discussed in the jurisprudence of international courts and tribunals and their relationship with other akin terms such as ‘customary’ and ‘traditional’ fishing rights. Issues related to the formation and establishment of these rights are also explored. The article also examines whether these rights have been superseded by the LOSC in the territorial sea and exclusive economic zone (EEZ), or whether they are still relevant in the post-LOSC era and maritime zones. It finally explores the relationship between historic fishing rights and treaty-based fishing access rights, and addresses the question posed by Brexit whether withdrawal from a fishing access treaty can unilaterally eliminate any related pre-existing historic rights. This discussion will provide an answer to the question whether it is possible for other states to have historic fishing rights in the UK’s territorial sea and EEZ following its withdrawal from the London Fisheries Convention and the European Union.