Ulva fasciata Delile is reported for the first time as an introduced species along the coast of the Baja California Peninsula. In 2002–2003, populations of U. fasciata were observed at only three sites (Monalisa Beach, El Faro Beach and mouth of Punta Banda Estuary) on the eastern side of Todos Santos Bay, Baja California, Mexico. The variation in mean thallus size suggests that this species attains maximum development during late summer and early autumn (September), gradually decreases during autumn–winter and disappears during spring (April). Reproductive plants were observed, especially in the warm months (summer–autumn), when seawater temperatures can reach 248C. A detailed review of specimens housed in Mexican herbaria revealed that U. fasciata is widely distributed along the Mexican Pacific coast, growing on rocks in the midtidal and upper intertidal zones. A detailed description of the vegetative and reproductive structures is provided.