The purpose of the present study is the understanding of Gentili's position on the law of the sea as expressed in his classic De iure belli (Hanoviae 1598). The key constitutive elements turn out to be: 1) the idea of the sea as 'res communis' to all mankind, which amounts to the concept of 'freedom of the sea'; 2) 'jurisdiction' of the coastal state on the adjacent sea, even on the high seas, in order to police crime and prevent/punish piracy. As such these two key elements, if taken in isolation, are rooted in the civil law tradition, but their true meaning can only be captured by placing them in the intellectual framework of which they constitute an integral part. Firstly, the epistemic structure of discourse hinges on the new science of natural jurisprudence, as applied to the subject of 'ius gentium bellicum'; secondly, the constituent theoretical languages emanate from a distinctive combination of civil law, scholastic-theological and humanist traditions. This procedure enables us to highlight the strikingly original and distinctly modern traits of Gentili's perspective on the law of the sea, which emerges as a corollary of his project of international/global order. A project that is based on the crucial notion of 'respublica magna' of mankind, a notion encompassing the two notions of 'freedom' and 'jurisdiction' that constitute and define the legal regime of the sea.The Stoic humanist notion of universal human society as 'corpus unum' implies, first of all, 'freedom of intercourse', or 'ius communicationis', to start from 'free passage' and 'freedom of commerce'. It is in the context of his argument about these basic freedoms that Gentili is finally led to discuss the subject of the 'law of the sea'. But, not only the concept of 'res communis', or 'freedom of use', but also the concept of 'jurisdiction', or 'protection', are strictly related to the same foundational concept of 'respublica magna'. This is a crucial characteristic of Gentili's approach to world order that is proved by reference to two cardinal points of his new 'cosmopolitan justice': 1) the 'international right to punish', as exemplified by the legitimacy of the wars of 'humanitarian intervention' and of the wars in support of the 'common law of mankind'; 2) the 'occupation of vacant land', which again underlines the relevance of the principle of 'jurisdiction' by striking a balance between the principle of 'free use' of nature and the 'jurisdiction' of the local ruler.Such a reconstruction definitely rejects the traditional image of Gentili as a supporter of 'maritime protectionism', on the ground of his Advocatio Hispanica published posthumously in 1613 and containing his pleadings as Spanish advocate before the Court of Admiralty. The guiding assumption here is that the book is strictly of a forensic character and as such devoid of any coherent theoretical substance. To the contrary, in his De iure belli, far from anticipating the English position in favour of 'mare clausum', Gentili tends to anticipate the essentials of Grotius' position, especially at the level of the ethic of the 'modern cosmopolis' and the related theme of colonial empire. On this very plane of discourse, Gentili's thinking transcends the so-called battle of the books and emerges as especially significant in illuminating the wider and deeper intellectual currents that contributed to the development of what were to become basic standard positions of modern international theory.