Macrobrachium carcinus is reported for the first time from the Cayman Islands. Specimens were collected on multiple occasions from an unusual habitat for this species – an anchialine cave. It is likely the cave was colonized by M. carcinus larvae via movement through subterranean karst tunnels from the sea inland to the cave entrance. Because only juveniles were collected, it is uncertain whether reproduction is occurring in this cave. However, it is possible M. carcinus may continue to colonize this and other similar caves, and sustained populations may eventually become established on Grand Cayman.