Here we report the isolation and structure elucidation of a new phytotoxic neo-clerodane diterpenoid, royleanumin (1), along with known constituents, ursolic acid and beta-amyrin. Its structure was deduced by spectral studies including NMR spectroscopy, mass spectrometric techniques and by the comparison with reported data. The compound was assayed for its phytotoxicity against lettuce seedlings. Royleanumin (1) exhibit strong activity and can be proposed as lead chemical for the development of a new environmentally friendly herbicide.