On 31 October 1631, the Spanish galleon Nuestra Señora del Juncal succumbed to a storm and sank near the coasts of Campeche (Mexico). From a legal perspective, Mexico and Spain retain a legal interest over the wreck as coastal state and flag state, respectively. This article examines the legal situation of the wreck in the light of international law and bilateral instruments between Mexico and Spain. In a first section, this article examines the issue of the ownership by considering the Juncal as a Spanish state vessel vested with sovereign immunity. Yet, the article also argues that Spain transferred the ownership of the wreck to Mexico during the state succession of 1836. In a second section, the article examines the general obligation to protect and preserve the Juncal as underwater cultural heritage pursuant the 2001 UNESCO Convention on the Protection of Underwater Cultural Heritage. This research argues that the general obligation requires from Spain and Mexico a duty to cooperate and due diligence obligations.