|Born||8 March 1813|
Vang, Thy, Denmark
|Died||20 June 1897 (aged 84)|
|Resting place||Assistens Kirkegaard, Copenhagen|
|Alma mater||University of Copenhagen|
|Fields||Zoology, Botany, geology, Archaeology|
|Institutions||University of Copenhagen,|
|Doctoral students||Hans Christian Gram|
Born in Vang, Thy on 8 March 1813, he held a lectorate in mineralogy in Sorø until 1845  when he became a professor of zoology at the University of Copenhagen. He worked on a great many subjects, including cephalopods, and also in genetics, where he discovered the principle of the alternation of generations in some parasitic worms in 1842.
Steenstrup discovered (1842) the possibility of using the subfossils of the Postglacial as a means of interpreting climate changes and correlated vegetation change, which he called succession in the recent past. Two of Steenstrup's students, Christian Vaupell and Eugen Warming further developed this line of research.
During Charles Darwin's extensive study of barnacles (Cirripedia) between 1846 and 1854, he corresponded with Steenstrup, who sent him both information and specimens. Darwin returned the specimens in 1854, and by way of thanks also sent Steenstrup a box of specimens, with a letter listing the 77 species of cirripedia he had enclosed as a gift. The specimens were dispersed in the Natural History Museum of Denmark; in 2014 staff at the museum found the list, and were able to identify most of the specimens for a new exhibition of their best objects. When Darwin published his series of monographs, he included notes acknowledging his debt to the kindness of Professor Steenstrup for sending him specimens of both modern and fossil barnacles.
In 1842, at the age of 29, he was elected a member of the Kongelige Danske Videnskabernes Selskab. In 1857, he was elected a foreign member of the Royal Swedish Academy of Sciences. In 1862, he was elected as a member of the American Philosophical Society and in 1863 elected as member of the Royal Society.
He died on 20 June 1897 in Copenhagen.
- Brockhaus' Konversations-Lexikon. 14th ed., Leipzig, Berlin and Vienna 1894; Vol. 15, p. 219
- Cowles, Henry C. (1911) The causes of vegetational cycles. Annals of the Association of American Geographers, 1 (1): 3–20 
- "Biographical Etymology of Marine Organism Names. B" (biographies of scientists with names beginning "B"), Hans G. Hansson, TJärnö Marine Biological Laboratory, Göteborg University and Stockholm University, TMBL.gu.se webpage: TMBL-P-Etymol-B.
- Bertelsen, Martin (29 August 2014). "Danish museum discovers unique gift from Charles Darwin". Phys.org. Retrieved 21 July 2015.
- Darwin, Charles (1851). "A monograph of the sub-class Cirripedia, with figures of all the species. The Lepadidæ; or, pedunculated cirripedes". Darwin Online. p. 170.
- Darwin, Charles (1851). "A monograph on the fossil Lepadidæ, or pedunculated cirripedes of Great Britain". Darwin Online. p. 76.
- Jungersen, H.F.E. (1902). Steenstrup, Johannes Japetus Smith. in: Dansk biografisk lexikon, band XVI: p. 327
- "APS Member History". search.amphilsoc.org. Retrieved 2021-04-20.
- Steenstrup, J.J.S. (1842). Geognostisk-geologisk Undersögelse af Skovmoserne Vidnesdam- og Lillemose i det nordlige Sjelland, ledsaget af sammenlignende Bemærkninger hentede fra Danmarks Skov-, Kjær og Lyngmoser ialmindelighed. Det Kongelige Danske Videnskabernes Selskabs Naturvidenskabelige og Mathematiske Afhandlinger, ser. 4, 9: 17–120.
- Spärck, R. (1932). Japetus Steenstrup, pp. 115–119 in: Meisen, V. Prominent Danish Scientists through the Ages. University Library of Copenhagen 450th Anniversary. Levin & Munksgaard, Copenhagen.