Upplands Väsby is a bimunicipal locality and the seat of Upplands Väsby Municipality, Stockholm County, Sweden with 37,594 inhabitants in 2010. When the Stockholm-Uppsala railway was opened in the 1860s, a station was built near the mansion Väsby. Some industries were attracted to the spot and a settlement grew. The name of the town became Upplands Väsby.
That name was taken from the post office, which was called so in 1919, in order to distinguish it from several other places around the country, also named Väsby. The railway station, however, was called just Väsby until 1939. After the Swedish municipal reform of 1952, the town became the seat and namesake of a municipality. With close to 40,000 inhabitants Upplands Väsby is the 27th largest locality (tätort) in Sweden, having a favourable location between Stockholm and Arlanda Airport.