A sulfated polysaccharide purified from a brown alga Ecklonia cava, having high anticoagulant activity was investigated for its antiproliferative effect on murine colon carcinoma (CT-26), human leukemic monocyte lymphoma (U-937), human promyelocytic leukemia (HL-60), and mouse melanoma (B-16) cell lines. The sulfated polysaccharide isolated and purified from an enzymatic extract of E. cava had a good selective tumor cell growth inhibition effect; its effect on HL-60 and U-937 was especially promising. The IC50 value for the sulfated polysaccharide from E. cava (ECSP) on U-937 was 43.9 μg mL−1. The presence of the sample in the cell culture media stimulated the induction of apoptosis, revealed by nuclear staining with Hoechst 33342. The apoptosis induction was confirmed by the cell cycle analysis, while pronounced sub-G1 phase arrests of 9.5% and 13.8% were also clearly observed when the cells were treated at 15 and 30 μg mL−1 of ECSP in the U-937 cell line, respectively. After a 24-h incubation period, ECSP dose-dependently enhanced the DNA fragmentation on the U-937 cell line as observed in the agarose gel electrophoresis assay. To rule out the action mechanism of ECSP for its anticancer activity, some western blot analyses were conducted with several antibodies (caspase-7, caspase-8, Bax, Bcl-xL, and PARP) and ECSP had a clear effect on the caspase -7 and 8 which cleave protein substrates, including PARP, an inducer of apoptosis responsible for DNA cleavage. Moreover, ECSP controlled the cellular transmembrane molecules like Bax and Bcl-xL. Taken together, the above results demonstrate that the apoptosis for antiproliferative effect of ECSP was clearly induced on U-937 cells.