In 1862 a text of medieval Welsh law, attributed to the tenth-century king Hywel Dda (Hywel the Good), was cited as part of the defence in a case, Attorney General v Jones, concerning disputed foreshore rights in Anglesey, north Wales. This article aims to explain why and how Welsh law, effectively abolished by the Acts of Union of 1536–43, was deployed as evidence in the case and how far this marked a readiness to accommodate the distinctive legal heritage of Wales within the framework of the nineteenth-century common law. As well as analysing the legal arguments presented, the article seeks to assess the broader significance of the case by setting it in the contexts of the Crown's increasingly vigorous claims to foreshores, the circumstances and attitudes of the real defendant, William Bulkeley Hughes, and antiquarian study of the medieval Welsh law-texts, including their use in previous mid-Victorian foreshore disputes.