Karl Cäsar von Leonhard (12 September 1779 – 23 January 1862) was a German mineralogist and geologist.

From 1797 he studied at the universities of Marburg and Göttingen, where Johann Friedrich Blumenbach was an important influence to his career.

He collected many mineralogical specimens on scientific excursions in Saxony and Thuringia, continued by travel to the Austrian Alps (including the Salzkammergut).

In 1818, through assistance from Baden minister of state Sigismund von Reitzenstein, he was appointed professor of mineralogy at the University of Heidelberg.

[5][6] Nowadays Leonhard is recognized as one of the key pioneers of loess studies[7] The term "leonhardite" bears his name, being defined as a partially dehydrated, opaque laumontite.