The Île Saint-Louis (French pronunciation: [il sɛ̃ lwi]) is one of two natural islands in the Seine river, in Paris, France (the other natural island is Île de la Cité, the Île aux Cygnes is artificial). The island is named after King Louis IX of France (Saint Louis).

The island is connected to the rest of Paris by bridges to both banks of the river and by the Pont Saint Louis to the Île de la Cité. This island was formerly used for the grazing of market cattle and stocking wood. One of France's first examples of urban planning, it was mapped and built from end to end during the 17th-century reigns of Henri IV and Louis XIII. A peaceful oasis of calm in the busy Paris centre, this island has only narrow one-way streets, no métro stations and two bus stops. Most of the island is residential, but there are several restaurants, shops, cafés and ice cream parlours at street level, as well as one large church, Saint-Louis-en-l'Île Church.