The tercio (Spanish pronunciation: [ˈterθjo], Portuguese: terço [ˈteɾsu], "third") was a Renaissance era military formation made up of a mixed infantry formation of about 3,000 pikemen, swordsmen and arquebusiers or musketeers in a mutually supportive formation. It was also sometimes referred to as the Spanish Square. It was widely adopted and dominated European battlefields in the sixteenth century and the first half of the seventeenth century.

The tercio was the product of the Italian Wars, in which the Spanish general, Gonzalo Fernández de Córdoba, reorganized the Spanish army throughout a series of conflicts at the end of the 15th century and early 16th century, into a tactically unique combination of combined arms centered around armored infantry. At first the army consisted of units of around 6000 men, called coronelias, which by 1534 had been reduced to the tercios of 3000, for increased mobility on the offensive. Armies using the tercio generally intended to field them in brigades of at least three, with one tercio in the front and two behind, the rearward formations echeloned off on either side so that all three resembled a stepped pyramid, hence the name tercio, which means "one third" (that is, one third of the whole brigade or battle group).