Karl Eckstein (28 December 1859, Grünberg, Silesia-22 April 1939 Ragusa, Dalmatia) was a German entomologist who specialised in Lepidoptera.
Karl Eckstein was a professor of Forest Zoology at the Forestry Academy in Eberswalde. He wrote Die Schmetterlinge Deutschlands mit besonderer Berücksichtigung ihrer Biologie und wirtschaftlichen Bedeutung 5. Band. Stuttgart which included Die Kleinschmetterlinge Deutschlands(1913–1933).