Abstract The brown macroalga Cladostephus spongiosus is reported for the first time from the central Pacific coast of the Baja California Peninsula. Thalli ranged from 3 to 20 cm in length, with the shortest occurring in January (winter) and the tallest in June (summer). Reproductive individuals were present in all seasons except autumn, when mean seawater temperatures reached 20.4°C. The species was found from the mid-intertidal to 6 m depth in the subtidal zone. The most common species growing with C. spongiosus were the brown algae Dictyopteris spp., Dictyota coriacea, and Padina caulescens, the red alga Polysiphonia johnstonii var. concinna and the surf-grass Phyllospadix torreyi. A detailed description of the vegetative and reproductive structures is provided. Its limited local distribution suggests a recent introduction to this region.