John Lionel Simonsen (22 July 1884, Levenshulme, Manchester – 20 February 1957, London) was an English organic chemist. Simonsen, whose parents came from Denmark (his father was a merchant), attended the Manchester Grammar School and studied at the University of Manchester, where he obtained his bachelor's degree with top grades in 1904, a Ph.D. in 1909 and was a student of William Henry Perkin Jr. In 1907 he became an assistant lecturer and demonstrator in Manchester. Before the First World War he was in Madras, where from 1910 he was professor colleague of Charles Gibson at the Presidency College. While Gibson went to England after the war broke out, Simonsen stayed in India, where he was an oil controller and advisor to the Indian Munitions Board. He also founded the Indian Science Cong