Mamalia wa majini ni mamalia ambao wanaishi na kupata riziki zao (zote au sehemu) majini, ama baharini ama katika maziwa, mito. Ni wanyama wa spishi mbalimbali (129) jamii ya mamalia ambazo hazihusiani kwa asili, ila zimefuata njia zinazofanana kidogo katika kuzoea kwa kiasi tofauti mazingira hayo badala ya kuendelea kuishi katika nchi kavu. Mamalia wa baharini Kaschner, K.; Tittensor, D. P.; Ready, J.; Gerrodette, T.; Worm, B. (2011). "Current and Future Patterns of Global Marine Mammal Biodiversity". PLoS ONE 6 (5): e19653. doi:10.1371/journal.pone.0019653 . PMC 3100303 . PMID 21625431 . Pompa, S.; Ehrlich, P. R.; Ceballos, G. (2011-08-16). "Global distribution and conservation of marine mammals". Proceedings of the National Academy of Sciences 108 (33): 13600–13605. doi:10.1073/pnas.1101525108 . PMC 3158205 . PMID 21808012 . Perrin, W. F. (2009). [[[:Kigezo:Google books]] Encyclopedia of Marine Mammals], 2nd, San Diego: Academic Press. ISBN 978-0-0809-1993-5.