Festucalex prolixus is a species of marine fish of the family Syngnathidae. It is found in the Western Central Pacific, from the Sulu-Celebes Sea and around west Papua New Guinea, Indonesia, including Cenderawasih Bay. Known specimens come from trawls of , so it is thought to inhabit waters deeper than . Very little is known about this species, but it is expected to be ovoviviparous and to feed on small crustaceans.