The origin of Santiago do Cacém remounts to pre-history, as it was attested by the archaeological researches in Castelo Velho, at the top of a hill, east of the borough, where you find the Roman ruins of Miróbriga. That village from the end of neolithic was lather fortified by the Celts and in the 3rd and 2nd cent. b.C., it would already keep important relations with other towns in the south of the peninsula.
With the Romanization, Miróbriga (a Celtic word) grew and became the center of an agricultural and cattle breeding area, as proved by its vast hippodrome. It was also an important watering place (thermae [...]).
The region was invaded by the Germanies and its importance will have decreased since the 5th cent., at such a pass that the Moslem have founded a new castle, over the facing hill, at west, wich they called Cacém, probably because of some alcaide.
Taken by the Templars in 1157, the Moors reconquer it in 1185 but, in the following year, the Santiago friars win it back.
In 1191, the Moslem regain it, but in 1217 is definitely conquered by the Christians, and D. Afonso II confirms his father's donation to Santiago Order, wich explains its name.
Between 1325 and 1336, the castle belonged to the Byzantine princess D. Vetácia, a chambermaid and friend of the Queen Saint Elizabeth's, and then returned to the Santiago Order, until 1594, when Filipe II donated it to the Dukes of Aveiro. In 1759, its possession passed to the crown.
It is a municipality since 1512, when D. Manuel I coceded a "foral" (charter) to the place.

in: Santiago do Cacém, ed. da Região de Turismo da Costa Azul, 1991