Karl Ernst Claus, also known as Karl Klaus or Carl Claus (; 22 January 1796 – 24 March 1864), was a Russian chemist and naturalist of Baltic German origin. Claus was a professor at Kazan State University and a member of the Russian Academy of Sciences. He was primarily known as a chemist and discoverer of the chemical element ruthenium, which he named after his homeland of Russia, but also as one of the first scientists who applied quantitative methods in botany.