Eisenia arborea is an edible brown alga occasionally used as a folk medicine in gynecopathy in Japan. A new phlorotannin was isolated from the alga during our search for naturally occurring anti-allergic compounds from edible algae guided by the inhibitory effect on histamine release from rat basophile leukemia (RBL)-2H3 cells. The phlorotannin was called "phlorofucofuroeckol-B." Its structure was determined by spectral analysis and chemical conversion. This paper describes the isolation, structure elucidation, and inhibitory effect of phlorofucofuroeckol-B on histamine release.