Fucoxanthin in brown algae, such as Wakame, Kombu and Mozuki, is an oxygen-containing carotenoid xanthophyll and is highly bioavailable. Dietary fucoxanthin is found in the blood after being processed to fucoxanthinol by deacetylating hydrolysis in the intestinal epithelial cells. The pharmaceutical parameters in human subjects of a single dose of 47 μmol fucoxanthin are C_max, 44.2 nmol/L; T_max, 4 h; t_1/2, 7.0 h;and AUC_∞, 664 nmol/Lxh. Most of fucoxanthinol is excreted in urine at around 25 h after ingestion and produces no side effects. Even with such short-term residence time in the blood, fucoxanthinol exhibits diverse and significant biofunctions, such as cancer preventing action, anti-obesity effect, improvement of lipid metabolism, and antioxidant potency. In this review, the bioavailability and diverse biofunctions of dietary fucoxanthin will be described.