Girolamo Muzio or Mutio Justinopolitano (1496 in Padua, Republic of Venice – 1576 in Barberino Val d'Elsa, Grand Duchy of Tuscany) was an Italian author in defence of the vernacular Italian language against Latin.
In 1551 he published, along with other Italian poems, his Arte Poetica, in three books, composed in blank verse.
Besides letters, histories, moral treatises, he wrote several tracts against the Reformers, especially those of the Italian nation, who at that time were numerous.
As a counterbalance to the Protestant writers of ecclesiastical history, called the Magdeburg Centuriators, Muzio, in 1570, published a Roman Catholic history of the two first centuries, which made up in polemic zeal for what it wanted in sound erudition.
Muzio's works on the Italian language, published as the Battaglie per diffesa dell'italica lingua (1582), include a defence of the vernacular against claims for the superiority of Latin, and the Varchina, in which Muzio attacks Benedetto Varchi's pro-Florentine Ercolano while upholding his own ideal of an Italian learned from books.