Johannes Japetus Smith Steenstrup FRS(For) HFRSE (8 March 1813 – 20 June 1897) was a Danish zoologist, biologist, and professor.

Born in Vang, Thy on 8 March 1813, he held a lectorate in mineralogy in Sorø until 1845 [1] 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.

[3] During Charles Darwin's extensive study of barnacles (Cirripedia) between 1846 and 1854, he corresponded with Steenstrup, who sent him both information and specimens.

[4] 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[5] and fossil barnacles.