The lycée Saint-Louis is a post-secondary school located in the 6th arrondissement of Paris, in the Latin Quarter. It is the only public French lycée exclusively dedicated to classes préparatoires aux grandes écoles (CPGE, the preparatory classes for the Grandes Écoles such as Ecole Polytechnique, CentraleSupelec, ESSEC Business School or HEC Paris). It is known for the quality of its teaching and the results it achieves in their intensely competitive entrance examinations (concours). It is widely regarded as one of the best preparatory class in France and one of the most elitist and prestigious along with his neighbours Lycée Henri IV or Lycée Louis-Le-Grand.

It is one of a very small group of secondary schools worldwide that can claim to have educated four or more Nobel laureates.