Martensia martensi is a species of amphipod crustacean, and the only species in the genus Martensia. It occurs in waters around Svalbard at depths of .
M. martensi is only known from four specimens collected in 1861 in two fjords on the island of Spitsbergen â Kongsfjorden and Isfjorden. They are all immature, and measure long.
A. Goës described Lysianassa martensi as a new species in 1866. It was redescribed in 1968, and tentatively assigned to the genus Uristes. In 1991, J. Laurens Barnard and G. S. Karaman erected a new genus, Martensia, for this species alone, and the genus remains monotypic.