The Republic of Massa (Italian: Repubblica di Massa) was a small Italian state located in Central Italy that existed from 1225 to 1336. It was founded in today's city of Massa Marittima and expanded to cover an area corresponding to the current upper Maremma.

It represented a commercial power of regional level, through its thriving mining district, thanks to the copper, alum and silver deposits in which its territory was rich.

The growth of the town of Massa Marittima is due to the relocation of the populonese bishopric to the city. The ancient city of Populonia mainly linked its economy to the processing of raw iron, transported by ship from the mines of the Island of Elba, since the Etruscan and Roman periods. However, due to the increase in pirate raids and the concomitant development of mining activities in the hinterland, Populonia entered a period of heavy decline which led to a change in the territorial balance in the area. The city of Massa benefited greatly from this centuries-old process and, from the 11th century, it witnessed the movement of the episcopal see within its walls.

Thanks to its strategic position it was in fact possible to control a large area of the Colline Metallifere rich in precious metals. This fortunate feature allowed the development of the village at the foot of the castle of Monteregio, an elegant domain of the bishop, and in the area of the square where today the major city buildings and the Cathedral stand, dedicated to San Cerbone, ancient bishop of Populonia.

Following a series of donations from castles and lands close to the city, the temporal power of the bishopric increased and from 1196 the bishops began to call themselves "Princes of Massa".^{[citation needed]}

The advent of the thirteenth century opened a flourishing period for Massa Marittima, which saw its population grow rapidly, thanks to an improvement in living conditions, greater safety of the mines and increased wealth of the inhabitants.^{[citation needed]}

On 21 April 1216, by the will of the bishop-prince Albert II, Massa Marittima swears allegiance to Pisa to enjoy his military protection.^{[citation needed]}

This great economic rise, driven in particular by the resumption of mining activities, in conjunction with a progressive debt of the bishops, made possible the birth of the community of Massa, in the form of a city republic. With prior agreement between Bishop Alberto II and the Massetani of 31 July 1225, drawn up by the imperial notary Rolando, the prince-bishop publicly renounced his government over the city in exchange for the payment of the debt of six thousand pounds of Pisan money that he had contracted with Sienese lenders. On 1 November of the same year, the day of All Saints, while the Massetani in Montieri paid a thousand silver marks of Massa to Ranieri di Raullo and his companions from Siena to settle the debt of the bishop, the Vicedomino Sigerio of Orlandino Galleana was named first mayor of the Republic.^{[citation needed]}