city and capital of Västmanland län (county), east-central Sweden. It lies at the confluence of the Svartån River and Lake Mälar (Mälar, Lake), west of Stockholm.

Västerås is Sweden's largest inland port and the centre of its electrical industry. Originally known as Aros (“River Mouth”) and later as Västra (“West”) Aros, it was already a market centre and a bishopric by the beginning of the Middle Ages. In medieval times it also served as a major export harbour for iron and copper from the Bergslagen mining region. Several national parliaments were held there, including that of 1527, which formally introduced the Reformation into Sweden, and that of 1544, which established the hereditary rights of the Vasa family to the throne. In the city's Gothic cathedral lie the remains of Eric XIV. Beside the Svartån River stands a 12th-century castle, now a museum. Västerås is home to a campus of Mälardalen University. Pop. (2005 est.) mun., 131,934.