Unmanned maritime systems (UMSs) comprise an important subcategory of unmanned military devices. While much of the normative debate concerning the use of unmanned aerial and land-based devices applies equally to those employed on or under water, UMS present unique challenges in understanding the application of existing law. This article summarizes the technological state of the art before considering, in turn, the legal status of UMSs, particularly under the UN Convention on the Law of the Sea (UNCLOS), and the regulation of their use under the law of naval warfare. It is not yet clear if UMSs enjoy status as ships under UNCLOS; even if they do, it is unlikely that they can be classified as warships. Nevertheless, their lawful use is not necessarily precluded in either peacetime or armed conflict.