This article deals with the doctrine of diplomatic immunity elaborated by the French jurist and humanist Pierre Ayrault (1536–1601). After a brief outline of the debate on diplomatic immunity from the end of the Middle Ages to the middle of the sixteenth century, the article focuses on Ayrault’s discussion of the topic in the works he published between 1563 and 1588, and points to some important changes that occurred in the way in which the figure of the ambassador was conceived at that time. These changes offer us a number of elements which could lead to a better understanding of the transition from the medieval to the early-modern conceptions of diplomacy and ius gentium.