The Lycée Léonin (Greek: Λεόντειο Λύκειο, romanized: Leónteio Lýkeio) is a non-profit private school in Athens, Greece.
The school has campuses in Nea Smyrni and Patissia and is directed by a community of six Marist Brothers that reside in Athens.
On July 25, 1838, approximately 14 months from the official opening of the Kapodistrian University of Athens, the Catholic priest Constantine Sargologos was granted, "on the order of the Ecclesiastic and Public Education of Royal Secretariat Territory", authorization for the founding of a primary school for boys.
In 1916, Lycée Léonin expanded to Patras, and the "St. Andrew" School was founded, which functioned until the end of World War II in 1945.
In the beginning of the 1960s, as Lycée Léonin at Sina Street had already 700 students in primary and high school, the building was deemed insufficient and the construction of a new campus in Nea Smyrni was decided and was completed in September 1962.