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 it began to grow in early autumn through winter, becoming matured in Spring, when the sea water temperature was 11.6–15.2 °C (53–59 °F) in average. Also called "devil weed", this species has invaded the Eastern Pacific, beginning in Baja California and advancing north along the California coastline.