Lumazine and its 3-methyl and 1,3-dimethyl derivatives bearing 6-acyl substituents (acetyl, propionyl, β-methoxypropionyl, and β-hydroxypropionyl) were isolated from the marine polychaete Odontosyllis undecimdonta. As part of ongoing research on marine natural products, the metabolites of this luminous swimming polychaete were examined. Specimens were collected from Toyama Bay (late September to late October annually), lyophilized, ground, and treated with water until bioluminescence ceased. After evaporation to dryness in vacuo, the residue was extracted with methanol. Eight compounds (2-9) were isolated via silica gel column chromatography and successive thin-layer chromatography. Their structures were identified as 6-acyllumazines based on high-resolution mass spectrometry (HRMS), ¹³C and ¹H nuclear magnetic resonance (NMR) spectral data, and synthesis verification—synthetic 1,3-dimethyl-6-propionyllumazine, 6-(β-methoxypropionyl)-1,3-dimethyllumazine, and 6-acetyl-1,3-dimethyllumazine matched the natural products. The structures were determined as 1,3-dimethyl-6-propionyllumazine (2), 3-methyl-6-propionyllumazine (3), 6-propionyllumazine (4), 6-acetyl-1,3-dimethyllumazine (5), 6-(β-methoxypropionyl)-1,3-dimethyllumazine (6), 6-(β-methoxypropionyl)-3-methyllumazine (7), 6-(β-hydroxypropionyl)-1,3-dimethyllumazine (8), and 6-(β-hydroxypropionyl)-3-methyllumazine (9). This is the first report of lumazine derivatives with propionyl or acetyl substituents from a natural source. Whether these derivatives are related to Odontosyllis bioluminescence or biorhythm is unknown, and studies on their roles and other metabolites are underway.