Karl (Carl) Theodor Ernst von Siebold (16 February 1804 – 7 April 1885) was a German physiologist and zoologist. He was responsible for the introduction of the taxa Arthropoda and Rhizopoda, and for defining the taxon Protozoa specifically for single-celled organisms.
He was born at Würzburg, Bavaria, the son of a professor of obstetrics and a cousin (some say younger brother) to the naturalist and physician Philipp Franz von Siebold. Von Siebold studied medicine and science chiefly at the University of Berlin (under K. A. Rudolphi) and also at Göttingen (under Johann Friedrich Blumenbach), submitting a thesis on the metamorphosis of the salamander. In 1831 he began to practice medicine in Heilsberg, East Prussia (now Lidzbark Warmiński), moving in 1834 to Königsberg, and then in the same year to be Director of the Midwifery School in Danzig.
He became professor of zoology, comparative anatomy and veterinary science at Erlangen in 1840, professor of zoology and physiology at Freiburg in 1845, professor of physiology at Breslau in 1850, and professor of zoology and comparative anatomy at the Maximilians-Universität in Munich in 1853. In Munich, he later received the additional duties as professor of zoology and director of the zoological and zootomical cabinet.
He was elected a member of the Royal Swedish Academy of Sciences in 1856.