A phytochemical investigation of methanol extract from leaves of Pachyrhizus erosus (L.) Urban, a leguminous shrub distributed in Vietnam and other tropical and subtropical countries led to the isolation of a new prenylated chalcone, erosusone (1) and a new megastigmane glycoside epimer, 3-episedumoside F₁ (9), together with thirteen known compounds including flavonoids (2–6), a 3-benzoxepine lactone (7), a pyridine-4,5-diol derivative (8), megastigmanes and megastigmane glycosides (10–15). Their structures were elucidated by means of high resolution-electrospray ionization (HR-ESI)-MS, one dimensional (1D) and two-dimensional NMR (2D-NMR) spectroscopy as well as comparison with the data reported in the literature. The cytotoxic effects on LU-1 (lung carcinoma), HepG2 (hepatocellular carcinoma), and MCF7 (breast carcinoma) cell lines were assessed. Prenylated chalcones 1–2 and isoflavone 3 exhibited cytotoxicity against all tested cell lines with IC₅₀ values ranging from 22.04 to 45.03 µM.