The hamlets of the genus Hypoplectrus comprise a species flock of about 20 species found on coral reefs of the tropical western Atlantic Ocean, distinguished by complex color patterns and sharing mitochondrial DNA haplotypes within the Caribbean Sea (Gulf of Mexico and Florida-centered species are about 3% divergent in the COI marker). The species show a variety of biogeographic patterns, from widespread common species to relatively rare species limited to small parts of the Greater Caribbean region. We describe here a distinctive striped morph of barred hamlet, apparently limited to Fort-Liberté Bay in northeastern Haiti, as the new species Hypoplectrus liberte. The bay is relatively large and isolated, with a long and narrow opening. The markings of the new species differ from the widespread Caribbean Barred Hamlet, Hypoplectrus puella, to a similar degree as the two recently described allied northern species, which both have divergent mtDNA sequences. The marking pattern of the new species is more conspicuous and less colorful than the widespread H. puella, perhaps an adaptation to more turbid waters (also true for the Gulf of Mexico and Florida species). The mtDNA COI sequence of the new species is the same as that of the other species in the Caribbean species flock. This unusual microendemic species should be an invaluable subject for studying the microevolution of a species radiation. With the tiny population and the vulnerability of Fort-Liberté Bay to development and habitat degradation, this new species represents a critical extinction risk.