The cold dark interstellar Taurus Molecular Cloud One (TMC-1) is a rich source of acetylenic and polyacetylenic molecular species. As well as linear closed-shell molecules (H(C≡C)nCN) and symmetric rotors (CH3C≡CH, CH3CCN), several radicals (C≡CH, C≡CCN, (C≡C)2H) have also been identified1-5, many of which had not been studied previously in the laboratory. Whether the observed abundances can be understood in terms of purely gas-phase ion-molecule chemical schemes, which produce reasonable agreement for the simplest polyatomic species, is unclear6,7; alternative models involving the participate interstellar grains as catalysts or sources have also been suggested8,9. We now report the detection in TMC-1 of a new molecule, triearbon monoxide (C3O), whose pure rotational spectrum has only very recently been studied in the laboratory. As C3O is the first known interstellar carbon chain molecule to contain oxygen, its existence places an important new constraint on chemical schemes for cold interstellar clouds. In fact, the observed abundance of triearbon monoxide fits quite well into our model of galactochemistry.