Orsay (French pronunciation: [ɔʁsɛ] (listen)) is a commune in the Essonne department in Île-de-France in northern France. It is located in the southwestern suburbs of Paris, France, 20.7 km (12.9 mi) from the centre of Paris.

A fortified location of the Chevreuse valley since the 8th century and agricultural domain of wealthy and influential people, the development of Orsay is marked by the introduction of a railroad in the second half of the 18th century (today the RER B of which two stations are located in Orsay) and donations which allow the construction of a hospital still active to this day.

Orsay is the main home to the Paris-Saclay University. The university significantly shapes Orsay's economy as it employs about 10,000 academic workers. The city's economy is also centered on high technology, with several companies drawn to the area by the Paris-Saclay's research and development infrastructure.

Seat of the Orsay campus of Paris-Saclay University, crossed by the 118 national road, Orsay has become a residential city of the Paris metropolitan area and the Chevreuse valley, easily accessible and with a preserved environment at the heart of the Paris-Saclay scientific cluster with laboratories of the CNRS and the Curie Institute on its territory.