​SIERO, a town of northern Spain, in the province of Oviedo, on the river Nora, and on the Oviedo-Trifiesto railway. Pop. (1900) 22,503. Siero is in the centre of a fertile agricultural district, in which live-stock is extensively reared. There are coal mines in the neighbourhood, and the local industries include tanning and manufactures of soap, coarse linen and cloths.