Boulogne-Billancourt (French: [bulɔɲ bijɑ̃kuʁ] ; often colloquially simply Boulogne, until 1924 officially Boulogne-sur-Seine, [bulɔɲ syʁ sɛn], lit. 'Boulogne-on-Seine') is a wealthy and prestigious commune in the western inner suburbs of Paris, France, located 8.2 km (5 mi) from the centre of Paris at Notre Dame. It is a subprefecture of the Hauts-de-Seine department and thus the seat of the larger arrondissement of Boulogne-Billancourt. It is also part of the Métropole du Grand Paris. Boulogne-Billancourt includes one island in the Seine: Île Seguin.
Boulogne-Billancourt is one of the wealthiest regions in the Parisian area and in France. Formerly an important industrial site, it has successfully reconverted into business services and is now home to major communication companies headquartered in the Val de Seine business district.