Karl Eckstein (28 December 1859 in Grünberg, Silesia – 22 April 1939 in Ragusa, Dalmatia) was a German entomologist who specialised in Lepidoptera. He studied natural sciences at the University of Giessen, receiving his doctorate in 1884 with a thesis on rotatoria. In 1886, he became an assistant to Bernard Altum at the zoological institute of the Eberswalde Forestry Academy in Eberswalde. In 1890 he obtained his habilitation, and in 1900 succeeded Altum as professor of forest zoology at the Academy.