Campo Maior (European Portuguese pronunciation: [ˈkɐ̃pu mɐˈjɔɾ] ⓘ) is a municipality in the Portalegre District, Alentejo Region, Portugal.

[4] Campo Maior was certainly a Roman settlement – the ancient Muro Dam is close by – which was under control of the Moors for half a millennium.

In 1260, Bishop Friar Pedro Pérez, the Town Lord, granted the first charter (foral) to the inhabitants of Campo Maior.

King John I of Portugal and Constable Nuno Álvares Pereira led their armies to Campo and besieged the town for six weeks, finally occupying it at the end of 1388.

As a consequence, in the 16th century, the town's New Christian community was so numerous that it provided most of those accused of Judaism who were included in the Portuguese Inquisition's auto-da-fé that took place in nearby Évora.

The need to re-fortify the town, which had grown markedly outside the medieval perimeter during the previous three centuries, and the urgency to build a new fortified perimeter to defend the inhabitants of the "new town" from the incursions of the Castilian armies, forced the kings of Portugal to invest large amounts of money, and to send contingents of military engineers, specialized workers and even more non-specialized workers to strength Campo's defenses.

Upon crossing the breach which had been put on fire by the defenders with the help of a French Huguenot engineer, the Spanish Army suffered heavy casualties and retreated in defeat as a Portuguese relief force from Elvas was arriving.

On September 16, 1732, at 3 am, a storm hit the Armory, located in the castle's main tower, which stored 6000 arrobas of gunpowder and 5000 pieces of ammunition.

The town, which until then had been formed by a single freguesia (parish), was divided in 1766 into the present two – Our Lady of Expectação and Saint John the Baptist.

The uprising of Campo Maior against the Napoleonic invasion was successful due to the help from the Badajoz army, which then garrisoned the town for three years.