Nerillidae is a species-rich family of meiofaunal, mainly interstitial annelids. The genus Mesonerilla previously contained eight described species. We here describe six new species of Mesonerilla based on detailed morphological light and scanning electron microscopy studies. The new species were collected from coastal subtidal and tidal sediments, as well as from anchialine caves, in the Mediterranean, Canary Islands, Panamá, Trinidad and Tobago and the USA. All species of Mesonerilla contain nine segments, two-three antennae, two club-shaped palps, parapodial cirri and compound chaetae in most segments. New species are diagnosed by unique character traits and/or a unique combination of characters involving differences in shape of appendages, reproductive characters and glandular and ciliary patterns. Their validation is further supported by a molecular phylogeny including these and the majority of described species of Mesonerilla. With most of the diagnostic characters of the described species being hardly discernible, and with potential additional molecular identifiable species left undescribed, the genus Mesonerilla, as many other interstitial groups, show a high degree of cryptic diversity.