The Oranienstein Letters are a series of letters sent by William V, Prince of Orange in December 1801 from Schloss Oranienstein near Diez, Germany. William addressed them to 15 Orangist ex-regenten of the old Dutch Republic, whom he advised to no longer stay out of government. This meant that some of his instructions given in the Kew Letters, that urged resistance against the French–Batavian invasion, were no longer in effect. He and his son, William Frederick, also recognised the Batavian Republic as legitimate, and renounced their hereditary stadtholderate. These were preconditions set by First Consul Napoleon of the French Republic for compensation for the loss of their possessions in the Netherlands, that had been confiscated by the Batavian Republic.
William V only decreed these letters after much hesitations, and he would later refuse to accept the mediatised Fulda monastery and the Imperial Abbey of Corvey as compensation, but he agreed that William Frederick did, and thus became Prince of Nassau-Orange-Fulda.