A tercio, Spanish for "[a] third") was a military unit of the Spanish Army during the reign of the Catholic Monarchs of Spain and Habsburg Spain in the early modern period. They were the elite military units of the Spanish monarchy and the essential pieces of the powerful land forces of the Spanish Empire, sometimes also fighting with the navy.

The Spanish Army is the terrestrial army of the Spanish Armed Forces responsible for land-based military operations. It is one of the oldest active armies – dating back to the late 15th century.