Ergothioneine is an emergent factor in cellular redox biochemistry in humans and pathogenic bacteria. Broad consensus has formed around the idea that ergothioneine protects cells against reactive oxygen species. The recent discovery that anaerobic microorganisms make the same metabolite using oxygen-independent chemistry indicates that ergothioneine also plays physiological roles under anoxic conditions. In this report, we describe the crystal structure of the anaerobic ergothioneine biosynthetic enzyme EanB from green sulfur bacterium Chlorobium limicola. This enzyme catalyzes the oxidative sulfurization of N-alpha-trimethyl histidine. On the basis of structural and kinetic evidence, we describe the catalytic mechanism of this unusual C-S bond-forming reaction. Significant active-site conservation among distant EanB homologues suggests that the oxidative sulfurization of heterocyclic substrates may occur in a broad range of bacteria.