Karl Ernst Claus (also Karl Klaus or Carl Claus, , 23 January 1796 – 24 March 1864) was a Baltic German chemist and naturalist. 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, but also as one of the first scientists who applied quantitative methods in botany.