A new genus and species of the suborder Maricola, representing the first marine triclad living in freshwater within a cave, is described. These flatworms show specialised features, such as a lack of pigmentation and eyes, indicating their troglobitic condition. Until now, cave-dwelling triclads have only been found as representatives of the other two triclad lineages. The phylogenetic trees based on 18S rRNA and 28S rRNA genes showed their sequences to form a clade situated in a rather derived position within the Maricola. Combining maximum likelihood and Bayesian analyses along with morphological studies, we undertake the first integrative description of a marine triclad. The new genus is characterised by a secondary copulatory bursa and ovaries located behind the root of the pharynx. Nevertheless, its assignment to a family of Maricola was a difficult task especially due to the paucity of unique defining morphological features of their families. The new species is found in a semiarid environment that suffered marine transgressions and regressions during the Cretaceous, which may explain the presence of this originally marine species in a freshwater cave. The restricted distribution of the new species and recent environmental conditions detected at its type locality raise major concern for its conservation.