A platelet-aggregatory inhibitor was isolated from the 50% MeOH extract of Phyllanthus niruri L. leaf. Its structure was determined to be methyl brevifolincarboxylate on the basis of the ¹H-, ¹³C-NMR, and high-resolution mass spectral data. We compared the antiplatelet aggregatory effects of the constituent with adenosine, a well-known inhibitor of platelet aggregation. Platelet aggregation was induced by collagen or adenosine 5′-diphosphate as an activating agent; the extent of inhibition was monitored with a platelet aggregometer employing a laser-scattering method. The inhibitory effects of methyl brevifolincarboxylate were found to be as potent as adenosine that is known to act on an A_2A subtype receptor.