(April 26, 1478), unsuccessful plot to overthrow the Medici (Medici Family) rulers of Florence; (Florence) the most dramatic of all political opposition to the Medici family. The conspiracy was led by the rival Pazzi family of Florence.

In league with the Pazzi were Pope Sixtus IV and his nephew Girolamo Riario, who resented Lorenzo de' Medici's (Medici, Lorenzo de') efforts to thwart the consolidation of papal rule over the Romagna, a region in north-central Italy, and also the archbishop of Pisa, Francesco Salviati, whom Lorenzo had refused to recognize. An assassination attempt on the Medici brothers was made during mass at the Cathedral of Florence on April 26, 1478. Giuliano de' Medici was killed by Francesco Pazzi, but Lorenzo was able to defend himself and escaped only slightly wounded. Meanwhile, other conspirators tried to gain control of the government. But the people of Florence rallied to the Medici; the conspirators were ruthlessly pursued and many (including the archbishop of Pisa) were killed on the spot.

The failure of the conspiracy led directly to a two-year war with the papacy that was almost disastrous for Florence. But the most important effect was to strengthen the power of Lorenzo, who not only was rid of his most dangerous enemies but also was shown to have the solid support of the people.