Brétigny-sur-Orge (, literally Brétigny on Orge) is a commune in the southern suburbs of Paris, France, from the city center.
The name of the settlement is attested as Britiniacum in 1146, as Bretigniacum in 1548, and subsequently as Breteigny.
The origins of Brétigny start in the era of Roman Gaul. The etymology comes from the Latin britiniacum, which means "property of the Breton". The name certifies that this territory was during that period dedicated to a Breton, coming from (Great) Britain, during the period of Briton migration (6th century). Brétigny afterwards belonged to the royal estate of the Merovingian kings (from the 5th century until the 8th century).
At the time of the creation of the commune, the city name was spelled without any accent. This accent was added in 1801, and the name of the river ("on Orge") was added in 1898.
The Hôtel de Ville was completed in 1864.
Brétigny-sur-Orge has an oceanic climate (Köppen climate classification Cfb). The average annual temperature in Brétigny-sur-Orge is . The average annual rainfall is with December as the wettest month. The temperatures are highest on average in July, at around , and lowest in January, at around . The highest temperature ever recorded in Brétigny-sur-Orge was on 25 July 2019; the coldest temperature ever recorded was on 16 January 1985.
It is served by Brétigny station on Paris RER line C.
On 12 July 2013 an intercity train derailed shortly after 17.00, killing 7 people.
Inhabitants of Brétigny-sur-Orge are known as Brétignolais.