Ioan C. Cantacuzino (Romanian pronunciation: [iˈo̯an kantakuziˈno], also Ion Cantacuzino; 25 November 1863 – 14 January 1934) was a renowned Romanian physician and bacteriologist, a professor at the Romanian School of Medicine and Pharmacy and a member of the Romanian Academy. He established the fields of microbiology and experimental medicine in Romania, and founded the Ioan Cantacuzino Institute.
Born in Bucharest as a member of the Cantacuzino family, he graduated from the University of Paris' Faculty of Sciences and Faculty of Medicine, and worked at several hospitals in Paris, obtaining his doctorate in 1894. Later in the same year, he began his academic career as a deputy professor at the University of Iaşi, and returned to Paris after two years to serve on the staff of the Pasteur Institute, where he worked under the direction of Ilya Ilyich Mechnikov.
In 1901, Cantacuzino was assigned a teaching position in Bucharest, where he became a major influence on a generation of scientists. His discoveries were relevant in the treatment of cholera, epidemic typhus, tuberculosis, and scarlet fever. As a disciple of Mechnikov, he devoted part of his research to expanding on the latter's field of interest (phagocytes, the body's means of defence against pathogens, as well as the issue of immunity and invertebrates). He invented the notion of contact immunity.