Siero is a municipality of the autonomous community of Asturias, in northern Spain. With a population of 52,194 as of 2023, Siero is the fourth most populous municipality in Asturias. It has a total of 29 parishes, with the largest being Lugones; its capital parish is La Pola Siero. It is located centrally within Asturias, and the northernmost point of Siero is about 10 km from Asturias' largest city, Gijón, and the shores of the Cantabrian Sea.