Two novel polyketides, spiculoic acids A (1) and B (2), have been isolated from extracts of the Caribbean marine sponge Plakortis angulospiculatus. The structures of 1 and 2 were elucidated by detailed analysis of spectroscopic data. Spiculoic acid A (1) showed in vitro cytotoxicity against human breast cancer MCF-7 cells. It has a putative polyketide biogenetic origin that involves incorporation of four butyrate units and a Diels Alderase catalyzed intramolecular [4 + 2] cycloaddition reaction.