Schack August Steenberg Krogh (November 15, 1874 – September 13, 1949) was a Danish professor with partly Romani background (Romani mother ) at the department of zoophysiology at the University of Copenhagen from 1916-1945. He contributed a number of fundamental discoveries within several fields of physiology, and is famous for developing the Krogh Principle.

In 1920 August Krogh was awarded the Nobel Prize in Physiology or Medicine for the discovery of the mechanism of regulation of the capillaries in skeletal muscle. Krogh was first to describe the adaptation of blood perfusion in muscle and other organs according to demands through opening and closing the arterioles and capillaries.

Krogh was a pioneer in comparative studies on animals. He wrote his thesis on the respiration through the skin and lungs in frogs: Respiratory Exchange of Animals, 1915. Later Krogh took on studies of water and electrolyte homeostasis of aquatic animals and he published the books: Osmotic Regulation (1939) and Comparative Physiology of Respiratory Mechanisms (1941). In addition Krogh contributed more than 200 research articles in international journals. He was a constructor of scientific instruments of which several had considerable practical importance, e.g. the spirometer and the apparatus for measuring basal metabolic rate.