Karl Friedrich Theodor Dahl (June 24, 1856 in north of Dahme, Holstein – June 29, 1929 in Greifswald) was a German zoologist, and in particular an arachnologist. The son of a farmer, Dahl studied at the universities of Leipzig, Freiburg, Berlin and Kiel. His dissertation (1884) was "Beiträge zur Kenntnis des Baus und der Funktion der Insektenbeine". He became a Privatdozent in 1887; with a habilitation thesis "Ueber die Cytheriden der westlichen Ostsee". Around this time he traveled to the Baltic states and (1896–1897) to the Bismarck Archipelago near New Guinea. He was also interested in biogeography.