The Cadaver Synod is the name commonly given to the ecclesiastical trial of Pope Formosus, who had been dead for about seven months, in the Basilica of St. John Lateran in Rome during January 897. The trial was conducted by Pope Stephen VI, the successor to Formosus' successor, Pope Boniface VI. Stephen had Formosus' corpse exhumed and brought to the papal court for judgment. He accused Formosus of perjury, of having acceded to the papacy illegally, and illegally presiding over more than one diocese at the same time. At the end of the trial, Formosus was pronounced guilty, and his papacy retroactively declared null.

Pope Formosus was the bishop of Rome and ruler of the Papal States from 6 October 891 until his death on 4 April 896. His reign as pope was troubled, marked by interventions in power struggles over the Patriarchate of Constantinople, the Kingdom of West Francia, and the Holy Roman Empire. Because he sided with Arnulf of Carinthia against Lambert of Spoleto, Formosus's remains were exhumed and put on trial in the Cadaver Synod. Several of his immediate successors were primarily preoccupied by the controversial legacy of his pontificate.