Adult female individuals of the nematode Mesomermis devii sp. nov. (Nematoda: Mermithidae) have been described. The host organism is not determined. The individuals were isolated from moist guano obtained in the dark zone of Devi Cave (West Georgia). The new species is characterized by the following: rounded and symmetrical apical end of the head; mouth aperture is located in the middle of the head terminus; 6 cephalic papillae are arranged in a single row; each papilla is equipped with 2 or 3 sensilla; stoma has uneven width and the pharyngeal tube is of even width along the whole length; small amphids, with large, slightly oval, roundish holes and large, rounded pouches; pear-shaped vagina. Additionally, the radial holes emerging from the gland situated in the cuticle of the terminal part of the tail differentiate it from the species of the genus Mesomermis. This is the second record of this genus in Georgia. A list of species in the genus Mesomermis distributed in the Palearctic and brief morphological descriptions indicating their hosts and distribution details are provided.