Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) is a toxic explosive known to be resistant to biodegradation. In this study, we found that sediment collected from two unexploded ordnance (UXO) disposal sites (UXO-3, UXO-5) and one nearby reference site (midref) in Hawaii contained anaerobic bacteria capable of removing HMX. Two groups of HMX-removing bacteria were found in UXO-5: group I contained aerotolerant anaerobes and microaerophiles, and group II contained facultative anaerobes. In UXO-3 and midref sediments, HMX-metabolizing bacteria were strictly anaerobic (group III and group IV). Using 16S rRNA sequencing, group I was assigned to a novel phylogenetic cluster of Clostridiales, and groups II and III were related to Paenibacillus and Tepidibacter of Firmicutes, respectively. Group IV bacteria were identified as Desulfovibrio of Deltaproteobacteria. Using [UL-14C]-HMX, group IV isolates were found to mineralize HMX (26.8% in 308 d) as determined by liberated 14CO2, but negligible mineralization was observed in groups I-III. Resting cells of isolates metabolized HMX to N2O and HCHO via the intermediary formation of 1-nitroso-octahydro-3,5,7-trinitro-1,3,5,7-tetrazocine together with methylenedinitramine. These experimental findings suggest that HMX biotransformation occurred either via initial denitration followed by ring cleavage or via reduction of one or more of the N-NO2 group(s) to the corresponding N-NO bond(s) prior to ring cleavage.