Castelnuovo Magra's origins can be traced back to the years between 1187 and 1203 when, according to Pelavicino codex (a chronicle reporting on the events in the Valle), Bishop Gualtiero erected a castle there. With this castle, Gualtiero intended to control a segment of the via Romea, important communication line towards north. The castle was then enlarged by Enrico da Fucecchio, interested in protecting further this area and its villas.

In 1306 Dante Alighieri mediated the peace between the Luni bishop and the Malaspina family, with a treaty which decreed the end of the ecclesiastical influence on the town. After a golden century in which Castelnuovo independently managed the power once belonged to the bishop, the town merged with Sarzana around 1470; Sarzana inhabitants welcomed the new allies as "borghesi et castellani" (citizens).