Two new species of isopod crustaceans belonging to the genus Stenosoma Leach, 1814 (Valvifera, Idoteidae) are described, illustrated, and compared with the other known species of the genus. The new species can be distinguished both from each other and from all their congeners by having a maxilliped with two coupling hooks; length of the appendix masculina on pleopod II; and an antennal flagellum with 12 articles in Stenosoma aaseni sp. nov.; and by having a pair of distinctive lateral spinous tubercles on pereonites I-II; and a pereon with a part of a mid-dorsal carina on the posterior half of each of the pereonites in Stenosoma inonuei sp. nov.; the two new species also differ in the morphology of mouthparts, pleotelson, and pereopods. Both new species were collected from seagrass meadows, colonized predominantly by Zostera (Zostera) marina Linnaeus, 1753 and Posidonia oceanica (Linnaeus) Delile, 1813, in the Sea of Marmara, Turkey. An up-to-date morphological identification key to species level for to genus Stenosoma is also provided.