This article reveals the existence of a hitherto unknown Black Chamber in the Dutch Republic and the identity of its principal codebreaker, Abel Tasien d'Alonne (1646–1723), acting also as private secretary to Grand Pensionary Heinsius. On the basis of an analysis of a number of previously unidentified worksheets, three cases are put together that merited d'Alonne's attention, one related to a French diplomat at the court of the Bavarian Elector during the years 1707–9, one related to an undercover agent of the Bavarian Elector in the Dutch Republic during the years 1711–12 and one related to a French emissary to the state council in Brussels during the period 1714–15. The emergence of a Black Chamber in The Hague is remarkable because the Dutch could always call upon the Black Chamber of Hanover for the solution of any intercepted, coded documents. This indicates that the development of inter-allied relations during the war played a more important role in the rise of the Dutch Black Chamber than one would expect.