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 ${sim}2.5-6.5$ fold greater inhibitory effects on arachidonic acid (AA) and U46619 induced platelet aggregation ($IC_{50}$: 19.9 and $99.8;{mu}M$; 20.3 and $53.8;{mu}M$; 25.9 and $123.4;{mu}M$, respectively) than acetylsalicylic acid (ASA, $IC_{50}$: 63.0 and $350.0;{mu}M$). Compound 2 was an ${sim}22-40$ fold stronger inhibitor than ASA on AA and U46619 induced aggregation ($IC_{50}$: 1.6 and $15.6;{mu}M$, respectively).