Gymnasium Leonhard, Historic school building in Basel, Switzerland

Gymnasium Leonhard is a language- and music-oriented secondary school at Kohlenberg in Basel, formed in 1997 by merging the Gymnasium am Kohlenberg and the Holbein-Gymnasium. Its building dates from 1884 and was expanded in 1906 to plans by Theodor Hünerwadel; the school was long known locally as the Affenkasten.

The school was established in 1813 as a girls' school and moved to its current building in 1884. It received federal recognition as a Maturitätsschule in 1913.

The school's popular nickname was "Affenkasten" (monkey box), a name remembered from its time as the Töchterschule. In 2013 it marked the jubilee "200 Jahre Höhere Mädchenbildung Basel", celebrating two centuries of higher girls' education in the city. Its alumni include the philosopher Andreas Urs Sommer and the archaeologist Auguste Bruckner (1919–1997).

The school stands on Kohlenberg in Basel, where the building erected in 1884 was expanded in 1906 to plans by Theodor Hünerwadel. It is a working secondary school, so the classrooms and corridors are not open to visitors; the site is best viewed from the street. The 1997 merger of the Kohlenberg and Holbein gymnasiums produced the present institution.

The school's partner schools include the Juglas-Gymnasium in Latvia, with contact since 1993 and annual exchanges since 1995, and the High School Affiliated to Fudan University in Shanghai, with contact since 2011 and a first exchange in 2012. The philosopher Andreas Urs Sommer is among the graduates of the Gymnasium am Kohlenberg.