Oskar Heinroth (1 March 1871 – 31 May 1945) was a Germanbiologist who was one of the first to apply the methods of comparative morphology to animal behaviour, and was thus one of the founders of ethology. His extensive studies of behaviour in the Anatidae (ducks and geese) showed that instinctive behaviour patterns correlated with taxonomic relationships determined on the basis of morphological features. He also rediscovered the phenomenon of imprinting, reported in the 19th century by Douglas Spalding but not followed up at the time. His results were popularised by Konrad Lorenz, whose mentor he was. Lorenz regarded Heinroth as the true founder of the study of animal behaviour seen as a branch of zoology.
Heinroth was born in Mainz-Kastel. He began his studies of duck and goose behaviour while working as a scientific assistant from 1898 to 1913. He subsequently became the director of the Berlin Aquarium, a post he held for more than 30 years. He was married to Katharina Heinroth née Berger, a herpetologist who worked at the Berlin Zoo. He died in Berlin on 31 May 1945.