Axon degeneration is one of the starting points for many neurodegenerative diseases including Parkinson’s and Alzheimer’s disease. According to the World Health Organization (WHO), by 2040, neurodegenerative disease will overtake cancer to become the second leading cause of death after cardiovascular disease. Recently, Sterile alpha- and armadillo-motif-containing protein (SARM1) has been linked to axon degeneration. SARM1 has three domains namely an armadillo repeat domain, followed by two tandem SAM domains, and a C-terminal Toll/interleukin-1 receptor (TIR) domain. Our studies show that the TIR domain of SARM1 has catalytic activity and can cleave NAD+ and NADP+, which are important co-factors for many biological processes including cellular respiration and maintaining energy at the cellular level. Depletion of NAD+ and NADP+ has been linked to axon degeneration. Furthermore, knockout of SARM1 in mice prevents axon degeneration following axon injury. To date, the key determinants for the regulation and activation of SARM1 are unknown. To characterize the active site and catalytic mechanism of the SARM1 TIR domain we determined the crystallographic structures of SARM1 TIR mutants. In addition, we assessed the role of residues surrounding a previously proposed activity site using site-direct mutagenesis and a fluorescent assay measuring εNAD cleavage. Overall, our results provide a structural basis for the NADase activity of SARM1.