2-Hydroxy-4-(1’-hydroxy-2’,6’,6’-triméthyl-4-oxo-cyclohex-2’-enyl)-but-3-enoic acid, a new megastigmane derivative, trivially named monanthosin (1), together with eight known compounds (chrysin (2), quercitrin (3), astilbin (4), heptulose (5), allantoin (6), heptitol (7), cis-N-p-coumaroyl tyramin (8) and trans-N-p-coumaroyl tyramin (9)) were isolated from the leaves of Monanthotaxis littoralis (Annonaceae). Structures were assigned by direct interpretation of their spectral data, mainly HR-TOFESIMS, 1D NMR (1H and 13C) and 2D NMR (1H-1H COSY, HSQC, HMBC and NOESY) and by comparison with reported values. The MeOH, EtOAc and n-BuOH extracts as well as compounds 1, 2, 4 and 8 exhibited variable antimicrobial and antioxidant activities. The ethyl acetate extract and compound 4 were the most active samples among extracts and compounds, respectively. The ethyl acetate extract and antibiotics (vancomycin and fluconazole) demonstrated synergistic effect against Escherichia coli, Pseudomonas aeruginosa, Candida albicans, Candida tropicalis and Cryptococcus neoformans and additive effect against Staphylococcus aureus.