More than 35 naturally occurring piericidins and closely related antibiotics have been reported, exhibiting diverse biological activities. In our search for new secondary metabolites from marine bacteria, we isolated a new cytotoxic piericidin derivative, glucopiericidin C (3), and for the first time as a natural product, 5-oxo-5-o-tolyl-pentanoic acid (8a), together with 10 known metabolites. The algal metabolite spatozoate (6) was also first isolated from bacteria. The crude extract of the marine-derived Streptomyces isolate B8112 showed moderate activity against Gram-positive/negative bacteria and Candida albicans, strong activity against Mucor miehei, and potent in vitro antitumor activity (mean IC70 < 10 μg ml⁻¹) in a six-cell line panel. Glucopiericidin C was structurally characterized using NMR and MS: it has the molecular formula C₃₀H₄₅NO₈, featuring a 5'-demethoxypiericidin A aglycone with a hexose moiety linked at the C-10 position. It exhibited antibacterial activity comparable to glucopiericidin A against Bacillus subtilis, Staphylococcus aureus, and Escherichia coli, activity against Chlorella vulgaris, and concentration-dependent cytotoxicity against 36 human tumor cell lines (mean IC50 2.0 μM, mean IC70 4.2 μM). A Compare Analysis with 100 reference compounds suggested a novel mode of action. Spatozoate was confirmed as a bacterial metabolite, and 5-oxo-5-o-tolyl-pentanoic acid as a natural product for the first time.