Grotius (1583–1645) is now widely acknowledged as an important figure in early modern contractual and consensual theories of political authority and legitimacy. However, as his thoughts on these debates are disseminated throughout his works rather than systematically ordained, it remains difficult to assess what, if anything, constitutes his distinctive mark. In the present paper, I will argue that his works contain a combination of two conceptual elements that have come to constitute a salient characteristic of early modern contract and consent theories: first, a strong obligation to keep one’s promises, and second, an account of perfect promises as transferrals of rights. In the political sphere, this means that citizens who have promised their obedience to the authorities are obligated to keep faith, which provides a solid foundation for political obligations. In addition, their promise implies that authorities receive the right to rule over them, which accounts for the legitimacy of these authorities’ power.