Manzamenones J (1) and K (2) and plakoridine B (3), three new oxylipins with unique carbon-skeletons related to 3,6-dioxo-4-docosenoic acid, were isolated from Okinawan marine sponges, Plakortis spp., and their structures elucidated on the basis of spectral and chemical means. Absolute stereochemisty of manzamenone A (4) was investigated by appling the modified Mosher's method developed recently for secondary carboxylic acids by Kusumi.