Cultivation of the endophytic fungus Chaetomium globosum, which was isolated from the inner tissue of the marine red alga Polysiphonia urceolata, resulted in the isolation of chaetopyranin (1), a new benzaldehyde secondary metabolite. Ten known compounds were also isolated, including two benzaldehyde congeners, 2-(2',3-epoxy-1',3'-heptadienyl)-6-hydroxy-5-(3-methyl-2-butenyl)benzaldehyde (2) and isotetrahydroauroglaucin (3), two anthraquinone derivatives, erythroglaucin (4) and parietin (5), five asperentin derivatives including asperentin (6, also known as cladosporin), 5'-hydroxy-asperentin-8-methylether (7), asperentin-8-methyl ether (8), 4'-hydroxyasperentin (9), and 5'-hydroxyasperentin (10), and the prenylated diketopiperazine congener neoechinulin A (11). The structures of these compounds were determined on the basis of their spectroscopic data analysis (1H, 13C, 1H-1H COSY, HMQC, and HMBC NMR, as well as low- and high-resolution mass experiments). To our knowledge, compound 1 represents the first example of a 2H-benzopyran derivative of marine algal-derived fungi as well as of the fungal genus Chaetomium. Each isolate was tested for its DPPH (1,1-diphenyl-2-picrylhydrazyl) radical-scavenging property. Compounds 1-4 were found to have moderate activity. Chaetopyranin (1) also exhibited moderate to weak cytotoxic activity toward several tumor cell lines.