Isolation and structure elucidation of three coumarins, murrayacoumarins A, B, and C, together with eight known coumarins, from the leaves of Murraya siamensis Craib collected in Thailand are described. Results of a primary screening of inhibitory effects of seven of these compounds on 12-O-tetradecanoylphorbol-13-acetate-induced Epstein–Barr virus early antigen activation in Raji cells are also presented.