We report initial observations of the morphology, phylogeny and ecology of a new species of Alvinocaridid shrimp that dominates the near-vent fauna at hydrothermal vents on the Mid-Cayman Spreading Centre in the Caribbean Sea. Specimens were collected from the world's deepest known vent field at a depth of 4960 m, and an off-axis vent field at a depth of 2300 m. Molecular phylogenetic data confirm that the specimens from these two sites are monospecific. Morphologically, the new species is consistent with the genus Rimicaris, but also has some features previously considered diagnostic for Chorocaris. Based on morphological and phylogenetic evidence, it is provisionally assigned to the genus Rimicaris. Its discovery extends the known geographic range of the Alvinocarididae in the Caribbean Sea and represents their deepest known occurrence. The two known populations of the species occur at sites that differ in depth by 2660 m and exhibit contrasting vent geochemistry, but are only 30 km apart within an isothermal water column. These features will allow future investigation of the influence of depth and local hydrography on population connectivity and gene flow.