The needs for building floating cities have been growing under the circumstances that global warming has led to the sea level rise and many coastal cities are facing the scarcity of land due to the urban crawl. Some island countries threatened by the sea level rise are engaged in the building of floating cities. The building of floating cities is more difficult than a way of reclamation and is also subject to the sea conditions. However, it is viewed that technology advances over years enable the building of floating cities highly feasible in technical terms. Nonetheless it appears that research efforts on legal issues concerning floating cities have not been paid much attention. The issues concerning the law of the sea need to be reviewed, since floating cities are built in the sea under the jurisdiction of the coastal state, unlike the land under a full sovereignty control of the coastal state. Since the UN Convention on the Law of the Sea and other treaties did not expect the emergence of floating cities when adopted, there exists a legal vacuum in international law. This is the same case in domestic legislations regulating buildings and installations employing floating structures. Although the 2016 amendment to the Building Act newly added the provision of ‘floating buildings,’ it is not sufficient to legally entitle and define rights and duties with regard to floating buildings and installations. UN Convention on the Law of the Sea needs to incorporate or a new international treaty needs to be adopted to accommodate the new form of maritime features. Domestically a special law that states the legal status of floating buildings need to be enacted.