Sezzadio is a comune (municipality) in the Province of Alessandria in the Italian region Piedmont, located about 80 kilometres (50 mi) southeast of Turin and about 15 kilometres (9 mi) south of Alessandria.

It is the location of the Abbey of Santa Giustina, founded in 722 by the Lombard king Liutprand.

In 1033 the abbey was enlarged by the marquis of Sezzadio, Oberto, remaining under the Benedictines until 1474.

The church has a large cotto façade divided by pilasters and crowned by Lombard bands.

argues that Antonio was the author of the Voynich cipher manuscript.