Carl Heinrich Apstein (19 September 1862, Stettin – 14 November 1950, Berlin) was a German zoologist (over a wide variety of animal life) and botanist (with a focus on phytoplankton and algae).
In 1889 he earned his doctorate from the University of Kiel with a dissertation on the spinnerets of the orb-weaver spider. Afterwards, he worked as an assistant to Karl Brandt (1854–1931) at the zoological institute in Kiel. As a young man he carried out studies of freshwater plankton in Holstein lakes (1890–95). In May 1898 he obtained his habilitation at Kiel for zoology and comparative anatomy, and a few months later took part as a zoologist in the Deutschen Tiefsee-Expedition (German Deep Sea Expedition) aboard the steamship "Valdivia".
In 1906 he was appointed associate professor in Kiel, and in 1911 became a scientific officer at the Preußischen Akademie der Wissenschaften in Berlin. In this position he worked as a publisher of scientific journals in the field of zoology, which included editorship of Das Tierreich ("The Animal Kingdom"; from 1927). He was a member of the International Commission on Zoological Nomenclature, and from 1918 to 1945 was secretary of the Deutschen Zoologischen Gesellschaft (German Zoological Society).
In addition to his research involving the 1898–99 Deutschen Tiefsee-Expedition, he was tasked with processing material taken from the Plankton-Expedition (1889) and the Deutschen Südpolar-Expedition (1901–03). In his investigations, Apstein distinguished himself in research of Thaliacea.