Norristown is a municipality in Montgomery County, Pennsylvania, United States, 6 miles (9.7 km) northwest of the city limits of Philadelphia, on the Schuylkill River. The population was 34,324 as of the 2010 census. It is the county seat of Montgomery County. Norristown is in a rich agricultural region; in the past, it had extensive manufactures of cigars, tacks, wire, screws, boilers, bolts, silos, tanks, iron, hosiery, knitting machines, underwear, shirts, lumber and milling machinery, paper boxes, rugs and carpets.Its official name is Municipality of Norristown, though it was formerly a borough operating under Pennsylvania's Borough Code and is frequently referred to as "the borough" even in statements by its officials. However, since 1986, Norristown has been governed under home rule charters, not under Pennsylvania's Borough Code. The 1986 charter was properly forwarded to the Commonwealth of Pennsylvania for inclusion in the Pennsylvania Code. Some areas outside the municipality, in the surrounding townships, also have "Norristown, PA" mailing addresses. The entire Municipality of Norristown is within the 19401 ZIP code.Norristown was incorporated as a borough in 1812, its population then being about 500. It was subsequently enlarged in 1853 and 1909. In 1900, 22,265 people lived here; in 1910, 27,875; in 1920, 32,319; and in 1940, 38,181, making Norristown the most populous borough in Pennsylvania. As of the 2010 census, the population was 34,324.