Formerly an Ancient Roman military camp, today's Sondrio was founded by the Lombards: in their language Sundrium meant "Exclusive property", referring to the status of free men (arimanni) of the holders of the city and the surrounding land.

The town is located in the middle of the province, and borders with the municipalities of Albosaggia, Caiolo, Castione Andevenno, Faedo Valtellino, Montagna in Valtellina, Spriana and Torre di Santa Maria.

Its hamlets (frazioni) are Arquino, Colda, Gualtieri, Ligari, Moroni, Mossini, Ponchiera, Sant'Anna, Sassella, Triangia and Triasso.

Other sights include the Torre Ligariana, once the collegiate's bell tower, and the Palazzo Pretorio, once the seat of the Grisons government.

Across the railway in direction of Milan there is the ancient Church of Maria della Sassella,[5] which the local tradition dates back to the 10th century.