Saint-Quentin-en-Yvelines [sɛ̃.kɑ̃.tɛ̃.ɑ̃n.iv.lin] is a new town and a communauté d'agglomération in the French département of Yvelines. It is one of the original five villes nouvelles (new towns) of Paris and was named after the Saint Quentin Pond, which was chosen to become the town's centre. The town was built from a greenfield site starting in the 1960s. In 2007, Saint-Quentin-en-Yvelines had a population of 146,598. It is part of the much larger Paris metropolitan area, and is around 25 km west of the centre of Paris.
The communauté d'agglomération is made up of 12 communes:
Of these communes, Montigny-le-Bretonneux is the most centrally located and has the largest population. The 2 most importants communes are Trappes and Montigny.
The population of the whole communauté d'agglomération has grown almost tenfold in the last 40 years, from a population of 15,118 in 1962.
Its president is Robert Cadalbert.
Every four years, Saint-Quentin-en-Yvelines serves as the start and finish point of the famous Paris–Brest–Paris bicycling endurance event.
Europcar has its head office in the business park of Val Saint-Quentin at Voisins-le-Bretonneux. It previously had its head office in the Immeuble Les Quadrants in Saint-Quentin-en-Yvelines.
At one time Bouygues had its head office in the Kevin Roche-designed Challenger complex in Saint-Quentin-en-Yvelines. It is now occupied by Bouygues Construction, one of the group's subsidiaries.