A new phenolic amide, (E)-3-(3,4-dihydroxyphenyl)-N-[2-(4-methoxyphenyl)-2-oxoethyl]-prop-2-enamide (1) was isolated from the stems of Cassia fistula. Its structure was elucidated by spectroscopic methods, including extensive 1D- and 2D-NMR techniques. Compound 1 was also evaluated for its anti-tobacco mosaic virus (anti-TMV) activity and it exhibit potential anti-TMV activity with inhibition rates of 16.2 %.