The main town in the upper Val Bormida, on the right branch of the Bormida river from which it takes its name. Millesimo partly retains its medieval aspect and is dominated by the ruins of a hilltop castle. Already inhabited in the Roman era, the town first appeared in tenth-century diplomas but later went into decline following frequent Saracen raids. In 1206 special privileges granted by the Del Carretto family led to an economic revival: the settlement was fortified and ringed with walls. It subsequently passed to Asti in 1260, the Visconti of Milan in the fifteenth century, and Spanish rule in 1659. The Savoy took possession on two occasions, in 1713 and 1736.
After the battle of Montenotte in 1796, Napoleon's troops defeated the Austrian and Piedmontese forces here, opening the road to Piedmont. Surviving medieval structures include the ruins of the Del Carretto Castle and, incorporated into the Villa Centurione, the remains of a Romanesque cloister from the Benedictine abbey of Santo Stefano (founded 1216). The parish church of Santa Maria extra Muros dates from the eleventh century with fifteenth-century alterations; it has a four-story campanile and an interior divided into a nave and two aisles. The medieval bridge over the Bormida is particularly notable — a twelfth-century span topped by a battlemented tower with two archways, added in the fourteenth century.
Other landmarks include the sixteenth-century Palazzo Del Carretto (now the town hall) and the shrine of the Madonna del Deserto, an eighteenth-century domed chapel rebuilt a century later. The nature reserve of Bric Tana lies just outside town. Cultural and sporting life concentrates in July with the "Luglio Millesimese" festival of concerts, theatre, and competitions.