Nicotinamide-containing cofactors play an essential role in many enzymes that catalyze two-electron redox reactions. However, it is difficult to engineer nicotinamide binding sites into proteins due to the extended nature of the cofactor-protein interface and the precise orientation of the nicotinamide moiety required for efficient electron transfer to or from the substrate. To address these challenges, we genetically encoded a noncanonical amino acid (ncAA) bearing a nicotinamide side chain in bacteria. This redox-active amino acid, termed Nic1, exhibits similar electrochemical properties to the natural cofactor nicotinamide adenine dinucleotide (NAD+). Nic1 can be reversibly reduced and oxidized using chemical reagents both free in solution and when incorporated into a model protein. This genetically encodable cofactor can be introduced into proteins in a site-specific fashion and may serve as a tool to study electron-transfer mechanisms in enzymes and to engineer redox-active proteins.