Synisoma albertoi sp. nov. (Isopoda: Valvifera: Idoteidae), is described and illustrated from specimens collected in the Strait of Gibraltar (southern Spain). The new species can be distinguished from all other species of the genus, with the exception of S. bellonae and S. raqueliae, by the fusion of the flagellar articles of the antenna. It is easily distinguishable from both S. bellonae and S. raqueliae by the presence of a tubercle on the cephalon; these three species also differ in the morphology of pleotelson, pereopods and appendix masculina. A key to the known species of Synisoma is provided.