The Franc affair (Hungarian: frankhamisítási botrány) was a plot by Hungarian nationalists to forge French bank notes. In the aftermath of World War I, Hungary lost a large part of its territory and population in a series of treaties its people considered unjust. This led many Hungarians to turn towards nationalism and revanchism. In 1922, Prince Lajos Windischgraetz was approached by Gyula Mészáros who presented him with a plan to counterfeit the French franc. Windischgraetz sought to damage the French economy while simultaneously raising funds for an internal coup in Hungary and irredentist activities. The plan came to halt when the notes produced by Mészáros were judged to be too primitive. In the summer of 1923, Windischgraetz met with German nationalist and retired Prussian Army General Erich Ludendorff who prompted him to utilize the forging equipment left from a similar unrealized German conspiracy.
The plot received widespread support in Hungarian nationalist circles including the patronage of high-ranking military and civilian officials. By September 1925, the plotters managed to produce 25,000 to 35,000 forged 1,000 franc banknotes. The conspirators began disseminating the banknotes in the Netherlands in December 1925, but were caught almost immediately. Twenty-four of the conspirators were tried in Budapest in May 1926. Most received light sentences in what is believed to have been a deliberate cover up by Hungarian Prime Minister István Bethlen. The affair facilitated the adoption of the International Convention for the Suppression of Counterfeiting Currency in April 1929 and formalized the role of the International Criminal Police Commission.