Bacteria belonging to the genus Clostridium are obligate anaerobes. Although bioinformatics analysis of Clostridium spp. genomes indicated genes for secondary metabolite biosynthesis, no secondary metabolites had been isolated from strictly anaerobic bacteria before. Herein we report the discovery of the first secondary metabolite from a strictly anaerobic bacterium, Clostridium cellulolyticum: closthioamide (C29H38N6O2S6), a hitherto fully unprecedented polythioamide. We induced its biosynthesis by adding aqueous soil extract (mimicking the bacterium's natural habitat) to the fermenter. The major metabolite was isolated, purified, and structurally elucidated as a symmetrical compound composed of diaminopropane, β-alanine, and p-hydroxybenzoyl units with six thioamide groups—an entirely polythioamidated metabolite is totally unprecedented among natural products. Closthioamide proved to be a novel antibiotic highly active against methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci (VRE) with a minimum inhibitory concentration (MIC) of 0.58 μM. Synthesis of the corresponding hexaoxa analogue (closamide) showed it was inactive, highlighting the importance of thioamide moieties for antibiotic activity. This is the first reported secondary metabolite from a strictly anaerobic bacterium, encouraging exploration of the underexplored metabolome of anaerobes.