Lex mercatoria is, as a label for contemporary transnational commercial law, well known from legal literature regarding international markets . Some arguments with respect to that concept have historical implications: a medieval body of commercial law is often considered as the predecessor of the lex mercatoria of today. Yet, legal historians have recently questioned whether a medieval commercial law existed in a uniform sense in different locations. As a result, the intellectual history of the concept of lex mercatoria is the more interesting. In this article, it is demonstrated that this notion was introduced in legal literature on international markets in the middle of the twentieth century, and that it was at that time borrowed from English legal-historical writings. The latter had taken a seventeenth-century concept as a description of historical facts, even though in the 1600s in England the late-medieval meanings of the term had fundamentally been changed. Originally, lex mercatoria described the procedure that was applied in tribunals of cities with market rights.