The present invention is a technology relating to a composition containing, as an active ingredient, an anthocyanin-fucoidan complex which is a product of ionic bonding of anthocyanin and fucoidan. More specifically, the anthocyanin-fucoidan complex, in which negative ions of fucoidan, a natural extract having high biocompatibility and biodegradability, are ionically bonded with positive ions of anthocyanin, enhances stability and solubility of anthocyanin even under acidic conditions inside the body, confirming the immune activity-enhancing effect of anthocyanin therein. Thus, provided is the anthocyanin-fucoidan complex as an immunity-boosting agent, an immuno-therapeutic anti-cancer agent, and an anti-cancer therapy adverse effect mitigating agent.