Dmitri Iosifovich Ivanovsky (alternative spelling Dmitrii or Dmitry Iwanowski; Russian: Дми́трий Ио́сифович Ивано́вский; 28 October 1864 – 20 June 1920) was a Russian botanist, the discoverer of viruses (1892) and one of the founders of virology.
Ivanovsky studied at the University of St Petersburg under Andrei Famintsyn in 1887, when he was sent to Ukraine and Bessarabia to investigate a tobacco disease causing great damage to plantations located there at the time. Three years later, he was assigned to look into a similar disease occurrence of tobacco plants, this time raging in the Crimea region. He discovered that both incidents of disease were caused by an extremely minuscule infectious agent, capable of permeating porcelain Chamberland filters, something which bacteria could never do. He described his findings in an article (1892) and a dissertation (1902). Then he worked in Warsaw and Rostov-on-Don.
In 1898, the Dutch microbiologist Martinus Beijerinck independently replicated Ivanovsky's experiments and became convinced that the filtered solution contained a new form of infectious agent, which he named virus. Beijerinck subsequently acknowledged Ivanovsky's priority of discovery.