In the present study, we investigated the in vitro and in vivo antitumor effects of a sulfated polysaccharide fraction from the brown alga Laminaria japonica (LJSP) on cervical carcinoma. In vitro, the results showed that LJSP exhibited the highest cell growth inhibitory effect on cervical carcinoma U14 cells among five tumor cell lines. In vivo, the results showed that LJSP could not only inhibit the growth of the tumor but also enhance the spleen and thymus indexes, as well as the body weight of U14 tumor-bearing mice. Moreover, the white blood cell count of H22 tumor-bearing mice showed no change in the LJSP-treated groups and little toxicological effects were observed on hepatic function and renal function in LJSP-treated mice bearing U14 tumor cells. Besides, LJSP induced apoptosis of transplanted tumor tissues by increasing the ratio of Bax/Bcl-2. These data showed that LJSP exhibited prominent antitumor activities and low toxic effects; thus, it could be developed to a safe and effective anticancer agent.