Ulvan is a sulfated polysaccharide from green algae with potent antitumor, antiviral, and immunomodulatory activities. However, no chemical synthesis of ulvan saccharides has been reported to date. In this paper, we performed the first efficient synthesis of the unique sulfated tetrasaccharide motif of type B ulvanobiuronic acid 3-sulfate. Based on the gold(i)-catalyzed glycosylation with glycosyl ynenoates as donors, efficient construction of the challenging α-(1 → 4)-glycosidic bonds between iduronic acid and rhamnose building blocks was achieved to afford the tetrasaccharide skeleton in a stereospecific manner. The synthetic sulfated tetrasaccharide was found to significantly improve the phagocytic activity of macrophage RAW264.7 cells.