On the basis of literature data and new morphological and ecological studies, the reinstatement of species rank for the Black Sea endemic species Cystoseira bosphorica is proposed. Previously reduced to a simple form of the Mediterranean endemic species C. crinita, this member of the caespitose Cystoseira group without spinose-like appendages clearly differs from C. crinita by having the axes long with smooth and prominent apices, the branches with numerous and slightly prominent cryptostomates, the aerocysts frequent, large, oval to subconical-broader at the apex, with 2 or more apical outgrowths, and by the receptacles long, cylindrical-tuberculate, simple or bifid, with apices blunt bearing sometimes a short lateral sub-apical spine. The species that shares the greatest number of characters with C. bosphorica is not C. crinita but the Mediterranean endemic species C. barbatula; however, this latter species clearly differs from C. bosphorica by the constant absence of aerocysts, the filiform branches with prominent and spaced cryptostomates, and by the receptacles smaller, compact, simple, humpy and subulate with sometimes 1–2 long lateral spine-like appendages.