Seaweed has been considered an indigestible food. Fucoidan, which is found abundantly in seaweed, especially in Cladosiphon okamuranus (Okinawa mozuku), has a high molecular weight and has been long believed to be hardly absorbed in the human digestive system due to a lack of certain digestive enzymes. We previously reported that fucoidan can be detected in serum and urine after oral intake of purified fucoidan in humans and rats. However, it is unclear whether the fucoidan in mozuku can be absorbed after digestion of mozuku. Therefore, we attempted to detect fucoidan in urine before and after mozuku intake. We determined the fucoidan concentration in urine after oral intake of Okinawa mozuku and urinary fucoidan was detected in several volunteers. In conclusion, these results suggest that fucoidan in mozuku can be absorbed after ingestion of mozuku.