Nuuanu beatricis n. sp., a new melitid amphipod, is described from shallow-water sandy bottoms on the S and SE coasts of Mallorca (Balearic Islands, Mediterranean). It differs from the rest of its congeners in the combination of: (1) non-geniculate antennule, much longer than antenna; (2) distal segment of mandibular palp shorter than second, armed with both D and E setae; (3) second segment of mandibular palp with 2 setae; (4) posteroventral lobe (sensu Lowry and Watson, 2002) absent from posterior margin of coxa 4; (5) reduced spine present on medial surface of coxa 5; (6) posterior margin of basis of pereopod 5 convex; (7) anterodistal lobe of coxa 7 wanting; (8) posterior margin of basis of pereopod 7 sparsely serrated along proximal half only; (9) pleonites 1-2 lacking dorsodistal spines; (10) anterodistal corner of epimeral plate 1 rounded; (11) exopod of uropod 3 longer than protopod; and (12) telson with terminal spine on each lobe. Although the new amphipod could be an alien species introduced from a distant tropical location with alien Caulerpa algae species, it is here regarded as an ancient Mediterranean element owing to its peracarid condition (i.e. reduced potential for dispersal), regressed visual apparatus, and crevicular-interstitial existence. The presumed presence of sternal gills in Nuuanu is rejected after demonstration of the coxal origin of the spiniform structures interpreted elsewhere as sternal gills.