VESTERAS or WESTERAS, a town and bishop's see of Swe den, capital of the district (lan) of Vestmanland, on a northern bay of Lake Malar, 6o m. N.W. by W. of Stockholm by rail. Pop. (1934), 31,229. The original name of the town was Vestra Aros ("western mouth"), in distinction from Ostra Aros, the former name of Uppsala. Several national diets were held in Vesteris, the most notable being those of 1527, when Gus tavus Vasa formally introduced the Reformation into Sweden, and 1544, when he had the Swedish throne declared hereditary in his family. Its Gothic cathedral, rebuilt by Birger Jarl
on an earlier site, and consecrated in 1271, was restored in 1850 6o, and again in 1896-98. The episcopal library contains the valu able collection of books which Oxenstjerna, the chancellor of Gus tavus Adolphus, brought away from Mainz near the end of the Thirty Years' War. A castle overlooking the town was captured by Gustavus Vasa and rebuilt by him, and again in the 17th cen tury, and remains the seat of the provincial Government.