This article examines the claim, made in numerous academic publications and even by the online Holocaust Encyclopaedia (run by the United States Holocaust Memorial Museum), that the Jewish residents of Zaragoza (Spain) were accused of ritually murdering a Christian child in 1182. After establishing that there is no medieval evidence to support the claim, this article traces its origins to a factual error made in a list of "Jewish ritual murders" featured in an anonymous French Antisemitic pamphlet printed in 1882. It then demonstrates how the claim was repeated, disseminated and publicized in subsequent lists of "Jewish ritual murders" produced by Antisemitic authors such as the Jesuit Father Rondina (1893), Justinas. B. Pranaitis (1894) and finally in the Nazi publication Der Stümer (1934). The leap from Antisemitic propaganda to unquestioned historical fact appears to have occurred when the respected historian Joshua Trachtenberg repeated it in his influential and well-respected 1943 book The Devil and the Jews. The Medieval Conception of the Jew and Its Relation to Modern Anti-Semitism.