The adults of Fortuynia atlantica sp. nov. are described in detail, including intraspecific variation of several morphological characters. Sensillus short, smooth, clavate; one pair of lamellar furrows; 14 pairs of notogastral setae vestigial c3; five pairs of genital setae, variations 6 5, 5 6, 5 4, 4 5; two pairs of anal setae, variations 3 2, 2 3. Leg setation (chaetome, solenidia): Leg I 1-4-2-3-18, 1-2-2; leg II 1-4-2-3-15, 1-1-1; leg III 2-3-1-3-15,1-1-0; leg IV1-2-2-3-13,0-1-0. The new species shows a remarkable secondary sexual dimorphism; males are characterized by a pair of rounded lateral notogastral protuberances, four large porose areas on notogaster, and very long, distally lanceolate notogastral setae la and lm, as well as by a less broad genital opening. The species colonizes the intertidal zone of rocky shores of the Bermuda Islands, where it lives mainly in crevices but also under covering mats of algae; it is absent from sandy beaches and mangrove trees. The species is well adapted to overflow with sea water. Feeding biology and other details in behaviour are discussed. It is the first described member of the family Fortuyniidae known from the Atlantic Ocean. This paper is listed as “Contribution #146, Bermuda Biodiversity Project, Bermuda Aquarium Museum & Zoo”.