Axon degeneration is an active program of self-destruction mediated by the protein SARM1. In healthy neurons, SARM1 is autoinhibited and, upon injury autoinhibition is relieved, activating the SARM1 enzyme to deplete NAD and induce axon degeneration. SARM1 forms a homomultimeric octamer with each monomer composed of an N-terminal autoinhibitory ARM domain, tandem SAM domains that mediate multimerization, and a C-terminal TIR domain encoding the NADase enzyme. Here we discovered multiple intramolecular and intermolecular domain interfaces required for SARM1 autoinhibition using peptide mapping and cryo-electron microscopy (cryo-EM). We identified a candidate autoinhibitory region by screening a panel of peptides derived from the SARM1 ARM domain, identifying a peptide mediating high-affinity inhibition of the SARM1 NADase. Mutation of residues in full-length SARM1 within the region encompassed by the peptide led to loss of autoinhibition, rendering SARM1 constitutively active and inducing spontaneous NAD and axon loss. The cryo-EM structure of SARM1 revealed 1) a compact autoinhibited SARM1 octamer in which the TIR domains are isolated and prevented from oligomerization and enzymatic activation and 2) multiple candidate autoinhibitory interfaces among the domains. Mutational analysis demonstrated that five distinct interfaces are required for autoinhibition, including intramolecular and intermolecular ARM-SAM interfaces, an intermolecular ARM-ARM interface, and two ARM-TIR interfaces formed between a single TIR and two distinct ARM domains. These autoinhibitory regions are not redundant, as point mutants in each led to constitutively active SARM1. These studies define the structural basis for SARM1 autoinhibition and may enable the development of SARM1 inhibitors that stabilize the autoinhibited state.