Fortezza Orsini, Medieval fortress and archaeological museum in Sorano, Italy.

Fortezza Orsini is a hilltop fortress in Sorano featuring two connected defensive towers with multiple levels of military architecture. Inside, an archaeological museum displays regional artifacts spanning different periods of history.

The fortress began as an Aldobrandeschi stronghold in the 12th century. At the end of the 13th century it passed by inheritance to the Orsini, who made it a residence and the seat of their county's power. It withstood sieges by Orvieto in the 14th century and by Siena in 1416–17 and 1454–55. In the 16th century it was enlarged, then became a Medici military post until decommissioning in the first half of the 18th century.

The name comes from the Orsini, who inherited the Aldobrandeschi castle at the end of the 13th century and made it, with Palazzo Orsini in Pitigliano, the seat of their county's power. The corner coat of arms combines the Aldobrandeschi rampant lions with the Orsini bars, roses and heraldic bear. Since the restoration the complex houses Sorano's civic archaeological museum, with medieval ceramics from the Giorgio and Erica Bartolomeo collection.

Visitors can explore underground passages, ancient ventilation shafts, and firing points across multiple levels to understand how the fortification worked. Sturdy footwear is recommended, as pathways are steep and sometimes narrow.

In the oldest core, restoration work brought to light 16th-century frescoes of the Sienese school; some, commissioned by Niccolò IV degli Orsini, depict Giovanni Boccaccio's song "Io mi son giovinetta" from the ninth day of the Decameron. A corner coat of arms on the San Pietro bastion combines the Aldobrandeschi lions with the Orsini bars, roses and bear.