Oskar Heinroth (Mainz-Kastel, 1 March 1871 – Berlin, 31 May 1945) was a German biologist who was one of the founders of ethology. Heinroth was director of the Berlin Zoo & Aquarium for over 30 years. He married twice, both times to colleagues: He was married to Magdalena (née Wiebe, 1904-1932) and, after her death, to Katharina (née Berger, 1933-1945).
His studies of behaviour in the Anatidae (ducks and geese) showed their instinctive behaviour patterns. He also rediscovered the phenomenon of imprinting, reported in the 19th century by Douglas Spalding but not followed up at the time. Heinroth showed that the behaviour of birds was characteristic of a species, as much as its anatomy.