Siero's two main rivers, which run throughout much of the municipality, are theNora and Noreña. Geographically,meadowlands make up a majority of the area within Siero. Like many other areas near the coast in Asturias, Siero tends to have a very temperate climate.[2] It alsoenclaves the municipality ofNoreña.
The area now known as Siero has been inhabited by humans since thePaleolithic period, especially near the current parishes of Argüelles, Lugones, andSamartindianes. TheRoman conquest of Iberia extended to the municipality, as evidenced by the numerousRoman bridges left over from that period.[3]
Siero would not be referenced in writing until 905, whenAlfonso III granted the church of San Martín de Siero to theOviedo Cathedral. Other churches in the municipality were also donated to the Oviedo Cathedral throughout that century. There are also records ofhospitals andleprosariums being constructed in the area 12th and 13th centuries; some of those continued operating until roughly the 18th century. On 14 August 1270,KingAlfonso X granted the area itsmunicipal charter, giving the area official recognition. Eighty years later, the region was split following the death ofAlfonso XI, withPedro I controlling Samartindianes and the rest of the region subject toEnrique II of Trastámara. Following Enrique II's ascent to thethrone of Castile, he exempted Siero from taxes due to its inhabitants' support during theCastilian Civil War.[3]
In the early 19th century, Siero was invaded multiple times by theFrench during theNapoleonic Wars, until the French withdrew from Asturias in 1812.[3] Throughout 20th century, due to the growth of thecoal mining industry in the region, Siero became one of the fastest growing municipalities in Asturias, more than doubling its population from the beginning of the century to the end. In modern times, thetertiary sector has become Siero's largest, withconstruction anddairy farming still remaining relevant.[2]
Siero currently consists of the following 29 parishes: