A new isoflavone, 5-O-methyl-2'-methoxy-3'-methylalpinumisoflavone (1), together with four known compounds 6-methoxy-5,7,4'-trihydroxyisoflavone (2), 6-methoxy-7,4'-dihydroxyisoflavone (3), (+)-marmesin (4), and 4-methylbenzoic acid carboxymethyl ester (5), were isolated from the mangrove endophytic fungus, Fusarium sp. (ZZF41). Their structures were determined by analysis of spectroscopic data. Compound 1 inhibited HEp-2 and HepG2 cells with IC50 values of 4 and 11 micromol/mL, respectively.