The final step in the biosynthesis of nicotinamide-adenine dinucleotide, a major coenzyme in cellular redox reactions and involved in intracellular signaling, is catalyzed by the enzyme nicotinamide mononucleotide adenylyltransferase (NMNAT). The X-ray structure of human NMNAT in complex with nicotinamide mononucleotide was solved by the single-wavelength anomalous dispersion method at a resolution of 2.9 Å. Human NMNAT is a symmetric hexamer whose subunit is formed by a large six-stranded parallel β-sheet with helices on both sides. Human NMNAT displays a different oligomerization compared to the archaeal enzyme. The protein–nicotinamide mononucleotide interaction pattern provides insight into ligand binding in the human enzyme.