Marine polychaetes Odontosyllis undecimdonta, commonly known as fireworms, emit bright blue-green bioluminescence. Until the recent identification of the Odontosyllis luciferase enzyme, little progress had been made toward characterizing the key components of this bioluminescence system. Here we present the biomolecular mechanisms of enzymatic (leading to light emission) and nonenzymatic (dark) oxidation pathways of newly described O. undecimdonta luciferin. Spectral studies, including 1D and 2D NMR spectroscopy, mass spectrometry, and X-ray diffraction, of isolated substances allowed us to characterize the luciferin as an unusual tricyclic sulfur-containing heterocycle. Odontosyllis luciferin does not share structural similarity with any other known luciferins. The structures of the Odontosyllis bioluminescent system’s low molecular weight components have enabled us to propose chemical transformation pathways for the enzymatic and nonspecific oxidation of luciferin.