Erich Harnack (10 October 1852, in Dorpat – 24 April 1915, in Halle an der Saale) was a German pharmacologist and toxicologist.
From 1869 he studied medicine at the University of Dorpat, receiving his doctorate in 1873 with the dissertation Zur Pathogenese und Therapie des Diabetes mellitus ("Pathogenesis and therapy regarding diabetes mellitus"). From 1873 he worked as an assistant at the pharmacological institute of the University of Strasbourg, and in 1877 obtained his habilitation. In 1880 he became an associate professor of pharmacology and physiological chemistry at the University of Halle, where in 1889 he attained a full professorship. In 1891 he founded an institute of pharmacology at the university. He is remembered for his pharmacological studies of physostigmine and apomorphine.
He was the son of theologian Theodosius Harnack and the brother of theologian Adolf von Harnack, mathematician Carl Gustav Axel Harnack and literary historian Otto Harnack.