Sargassum horneri is a species of brown macroalgae that is common along the coast of Japan and Korea. It is an annual algae which has a varying fertile season along the coast. In Wakasa Bay, Japan it begins to grow in early autumn through winter and matures in spring, when the sea water temperature is around 11.6–15.2 °C (53–59 °F). Also called "devil weed", S. horneri has invaded the Eastern Pacific, beginning in Baja California and advancing north along the California coastline.