Sargassum filicinum Harvey is an important ecological engineer in the subtidal regions of northeastern Asian waters, as it forms marine forests that provide shelter for invertebrates and fishes. First discovered in Long Beach Harbor, CA, USA, in 2003, S. filicinum was thought to occur mainly in the California Channel Islands and near Ensenada, Mexico. However, during a survey from 2007 to 2009, we found S. filicinum off Isla Natividad, which is an extension of its range 550 km south of its last recorded location in the Mexican Pacific. Morphological and molecular data support the identification of the species. Considering its faster spread from Ensenada to Isla Natividad (4 years) than Sargassum muticum (14 years), S. filicinum may follow the path of S. muticum and arrive at the subtropical-tropical Pacific coast of Mexico. Although S. muticum has become well established within the local seaweed flora, S. filicinum may replace native Sargassum species and change the composition of local marine floras.