Cosmarium bahianum sp. nov. is described from samples collected from a bromeliad tank (phytotelmata) in a “restinga” area located at Salvador, Bahia State, northeast Brazil. Cosmarium bahianum belongs to the group of placoderm desmids, and is mainly characterized by rather small, smooth-walled, but remarkably punctate cells that are elliptic to subtriangular in apical view, as well as by a globose to elliptical, smooth-walled zygospore. The new species was collected from Hohenbergia littoralis J.B.Smith, an endangered endemic bromeliad species from the Brazilian Restinga with distribution restricted to the north coast of Bahia. The relationship of C. bahianum with morphologically similar species is discussed.