The Syllabus of Errors is the name given to a document issued by the Holy See under Pope Pius IX on 8 December 1864, as an appendix to his encyclical letter Quanta cura. It condemns a total of 80 propositions that the Pope considered as errors or heresies.

Pope Pius IX was head of the Catholic Church from 1846 to 1878. His reign of 32 years is the second longest of any pope in history, behind that of Saint Peter. He was notable for convoking the First Vatican Council in 1868 and for permanently losing control of the Papal States in 1870 to the Kingdom of Italy. Thereafter, he refused to leave Vatican City, declaring himself a "prisoner in the Vatican".