The article discusses the stated reasons of the Vatican City State (Vatican) for declining German non-governmental organisation (NGO) Mission Lifeline’s request to act as the flag State of its search and rescue (SAR) vessel Lifeline from the perspective of the international law of the sea. In this respect, the article’s main purpose is not to frame these events as a novel challenge to the law of the sea as such, but to record a rare instance of relevant State practice of the Vatican and to place it within the broader context of the Vatican’s scarce and peculiar his-torical practice in maritime matters. The article first provides an overview of the situation of SAR vessels of NGOs at the time of Mission Lifeline’s request given the increasingly hostile attitude of coastal European Union Member States. In that regard, the article focuses on the issues of deregistration by flag States and the absence of States willing to grant their flag to SAR vessels of NGOs. Next, the article analyses the reasons given by the Vatican for declining Mission Lifeline’s request. In this respect, the article shows that the Vatican could legally act as the flag State of NGO vessels conducting SAR operations in the Mediterranean. In particular, the article highlights the issues of the existence of a “genuine link”, flag State responsibilities, and vessel immunity in the light of the Vatican’s unique historical and political situation. The conclusion raises some further legal and practical challenges that the Vatican would face if it chose to grant such vessels its flag.