Poissy is a commune in the Yvelines department in the Île-de-France region in north-central France. It is located in the western suburbs of Paris, 23.8 km from the centre of Paris. Inhabitants are called Pisciacais in French. Poissy is one of the oldest royal cities of Île-de-France, birthplace of Louis IX of France and Philip III of France, before being supplanted from the 15th century by Saint-Germain-en-Laye.