Buckwheat is a healthy food commonly eaten worldwide. The antitumor activity of buckwheat polysaccharides (BWPSs) has not yet been evaluated. In recent years, inducing differentiation of leukemic cells has become one of the most important therapeutic approaches for curing leukemia, and this strategy effectively inhibits leukemia cell proliferation and growth because the differentiation inducer changes leukemic cell morphology and cellular characters by inducing cellular maturity. The ability of BWPS to induce the differentiation of human leukemic THP-1 cells (monocyte [MNC]/macrophage-like cells) was investigated by both direct and indirect treatments in this study. In the indirect treatment, BWPS significantly stimulated cytokine secretion (differentiation inducer) in MNCs from peripheral blood mononuclear cells in MNC-conditioned medium (BWPS-MNC-CM) following a 24-hour treatment, and THP-1 cell differentiation and maturity were significantly increased after 5 days of treatment with the BWPS-MNC-CM. On the other hand, BWPS directly induced THP-1 cell differentiation and maturity following 3-day and 5-day treatments in a dose-dependent manner and exerted phagocytic activity and superoxide anion production in these mature cells. These findings indicate that BWPS has potential for differentiation therapy in leukemia.