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Hauts-de-Seine is a department in the Île-de-France region of north-central France. Boulogne-Billancourt is a city and commune in the Hauts-de-Seine department, of which it is a subprefecture. With a population of over 120,000, Boulogne-Billancourt is the most populous suburb in the Paris metropolitan area. The city is located on the western edge of Paris, on the south bank of the Seine River.