Tropodithietic acid is a monocarboxylic acid that is 8,9-dithiabicyclo[5.2.0]nona-1,3,6-triene substituted by a carboxy group at position 2 and an oxo group at position 3. It is a broad spectrum antibiotic produced by the marine bacterium Phaeobacter inhibens that also acts as a signalling molecule at lower concentrations. It has a role as a toxin, an antibacterial agent, a signalling molecule, a marine metabolite and a bacterial metabolite. It is a monocarboxylic acid, an organic heterobicyclic compound, an organic disulfide, an organosulfur heterocyclic compound and a cyclic ketone. It is a tautomer of a thiotropocin.