Karl Gottlieb Grell (28 December 1912, Burg an der Wupper – 4 October 1994) was a German zoologist and protistologist, known for his work on Trichoplax.
Karl Grell received his doctorate (Promotion) in 1934 from the University of Bonn, for a dissertation on the digestive tract of the common scorpionfly (Panorpa communis). Subsequently, he worked primarily on unicellular eukaryotes and the metazoan Placozoa. During World War II, he was assigned to an anti-malarial unit in southeast Europe.