A methanol extract of Sophora japonica was subjected to anti-platelet activity guided fractionation affording the isolation of four flavonoids and six flavonoid-glycosides: biochanin A (1), irisolidone (2), genistein (3), sissotrin (4), sophorabioside (5), genistin (6), tectoridin (7), apigenin (8), quercitrin (9), and rutin (10). The structure of each compound was determined by a variety of spectroscopic methods. Among the compounds, 1, 3, and 7 showed ∼2.5–6.5 fold greater inhibitory effects on arachidonic acid (AA) and U46619 induced platelet aggregation (IC₅₀: 19.9 and 99.8 μM; 20.3 and 53.8 μM; 25.9 and 123.4 μM, respectively) than acetylsalicylic acid (ASA, IC₅₀: 63.0 and 350.0 μM). Compound 2 was an ∼22–40 fold stronger inhibitor than ASA on AA and U46619 induced aggregation (IC₅₀: 1.6 and 15.6 μM, respectively).