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, a signalling molecule, a marine metabolite, an antibacterial agent and a bacterial metabolite. It is an organic disulfide, a cyclic ketone, an organosulfur heterocyclic compound, an organic heterobicyclic compound and a monocarboxylic acid. It is a tautomer of a thiotropocin.