Johannes Japetus Smith Steenstrup was born on March 8, 1813, in Vang Sogn, Hundborg Herred, Thisted County, Denmark.

He was a Danish zoologist, biologist, and professor. He was a lecturer in mineralogy in the Academy of Sorø until 1845 when he became a Professor of Zoology at the University of Copenhagen. He studied genetics and discovered the principle of the alternation of generations in some parasitic worms in 1842. He also discovered the possibility of using the subfossils of the Postglacial period as a means of interpreting climate and vegetation changes. Between 1846 and 1854, he corresponded with Charles Darwin (1809-1882) and, they exchanged both the information and specimens. The collections of these specimens are found today in the Natural History Museum of Denmark. Together with Johan Lange, Steenstrup was the publisher of Flora Danica, fasc. 44. In 1842, he was elected a member of the Royal Danish Academy of Sciences and Letters. In 1857, he was elected a foreign member of the Royal Swedish Academy of Sciences and, in 1862, the American Philosophical Society. He published numerous articles, papers, and books related to his field of study.

In 1841, he married Ida Margrethe Kaarsberg (1811–1882). He died on June 20, 1897, in Copenhagen, Denmark.