borough (town), seat of Montgomery county, southeastern Pennsylvania, U.S. It lies on the north bank of the Schuylkill River, 17 miles (27 km) northwest of Philadelphia and near the eastern terminus of the Pennsylvania Turnpike.

The site was purchased in 1704 by Isaac Norris and William Trent from William Penn (Penn, William), Jr. (son of Pennsylvania's founder), and was organized as a township in 1730. The land was acquired in 1776 by the College and Academy of Philadelphia. During the American Revolution the school was suspected of disloyalty, and the state assembly revoked its charter and transferred its assets to the newly created University of Pennsylvania (Pennsylvania, University of). When Montgomery county was created in 1784, Norristown (then The Town of Norris) became the county seat. Canalization of the Schuylkill and Delaware (Delaware River) rivers and completion of a railroad link (1834) to Philadelphia spurred the borough's development as an industrial centre that eventually produced chemicals, machinery, metal products, plastics, steel, rubber, and textiles.

Norristown's economy now is based on service activities with some light manufacturing, machinery manufacture, product assembly, and warehousing. Valley Forge, the encampment site for George Washington's army during the winter of 1777–78, is preserved nearby as a national historical park. Mill Grove, early home of the naturalist-artist John James Audubon (Audubon, John James), has been developed as a museum and wildlife sanctuary. Inc. borough, 1812. Pop. (1990) 30,749; (2000) 31,282.