The city lies 14 km from the Tyrrhenian Sea, in the Maremma, at the centre of an alluvial plain, on the Ombrone river.
It is the most populous city in Maremma, with 82,284 inhabitants. The comune of Grosseto includes the frazioni of Marina di Grosseto, the largest, Roselle, Principina a Mare, Principina Terra, Montepescali, Braccagni, Istia d’Ombrone, Batignano, Alberese and Rispescia.
The origins of Grosseto can be traced back to the High Middle Ages. It is first mentioned in 803 as a fief of the Counts Aldobrandeschi, in a document recording the assignment of the church of St. George to Ildebrando degli Aldobrandeschi, whose successors where counts of the Grossetana Mark until the end of the 12th century.
Grosseto steadily grew in importance owing to the decline of Rusellae and Vetulonia until it was one of the principal Tuscan cities. In 1137 the city was besieged by German troops, led by duke Henry X of Bavaria, sent by the emperor Lothair III to reinstate his authority over the Aldobrandeschi.
In the following year the bishopric of Roselle was transferred to Grosseto. In 1151 the citizens swore loyalty to the Siena, and in 1222 the Aldobrandeschi gave the Grossetani the right to have their own podestà together with three councilors and consuls.
In 1244 the city was reconquered by the Sienese, and its powers, together with all the Aldobrandeschi’s imperial privileges, were transferred to Siena by order of the imperial vicar. Thereafter Grosseto shared the fortunes of Siena.
It became an important stronghold, and the fortress (rocca), the walls and bastions can still to be seen. The walls are now used as a public park and walking area.