Cadaver Synod also called the Cadaver Trial  is the name commonly given to the posthumous ecclesiastical trial of Catholic Pope Formosus held in the Basilica of St. John Lateran in Rome during January of 897. The trial was conducted by the successor, Pope Stephen (VI) VII, to Formosus's successor, Pope Boniface VI. Stephen accused Formosus of perjury and of having acceded to the papacy illegally. At the end of the trial, Formosus was pronounced guilty and his papacy retroactively declared null. The Cadaver Synod is remembered as one of the most bizarre episodes in the history of the medievalpapacy.

Formosus was accused of transmigrating sees in violation of canon law, of perjury, and of serving as a bishop while actually a layman. Eventually, the corpse was found guilty. Liutprand and other sources say that, after having the corpse stripped of its papal vestments, Stephen then cruelly tortured the corpse by cutting off the three fingers of the right hand that it had used in life for blessings, next formally invalidating all of Formosus's acts and ordinations (including, ironically, his ordination of Stephen [VI] VII as bishop of Anagni). The body was finally interred in a graveyard for foreigners, only to be dug up once again, tied to weights, and cast into the Tiber River.

The macabre spectacle turned public opinion in Rome against Stephen. Rumors circulated that Formosus's body, after washing up on the banks of the Tiber, had begun to perform miracles. A public uprising led to Stephen being deposed and imprisoned. While in prison, in July or August 897, he was strangled.