Platelets play a crucial role in hemostasis, an essential physiological process that prevents excessive blood loss. In cases of hemorrhage, circulating platelets adhere to sites of injury in blood vessels, become activated upon exposure to adhesive proteins or soluble agonists, and form a platelet plug to achieve hemostasis. However, excessive platelet activation and aggregation can lead to thrombus formation and subsequent vessel occlusion. Korean dandelion (Taraxacum coreanum Nakai) is used in traditional Korean medicine to treat inflammatory diseases such as gastritis, gastric ulcer, and tonsillitis. In this study, the anti-platelet effects of an ethanol extract of T. coreanum (TCE) were investigated in vitro using collagen-, thrombin-, TPA-, or ADP-stimulated platelet aggregation. TCE significantly inhibited platelet aggregation induced by collagen, thrombin, and ADP. Furthermore, TCE exhibited obvious inhibitory effects on granule secretion, TXA2 production, integrin αIIbβ3 activation, and clot retraction. We found that TCE attenuated PI3K/Akt and MAPK (p38 and ERK) signaling pathways, and increased cAMP level. The data presented here demonstrate that TCE inhibited agonist-induced platelet aggregation and thrombus formation. These inhibitory effects may be associated with the inhibition of COX-1 activity, integrin αIIbβ3 activation, and increase of cAMP level. Therefore, we suggest that TCE may have therapeutic potential as an antiplatelet and antithrombotic agent.