The objective of this study was to investigate the influence of a commercially available orthotic device on rearfoot movement and peak impact force variables during running in combat assault boots. Eight military trainees performed running trials under two running conditions: boot with standard-issue insole and boot with the test orthotic. For each trial, vertical ground reaction force and frontal plane rearfoot angle data were collected. It was found that peak eversion angle was not significantly influenced by the orthotic device (p > 0.05), but that this peak occurred later in stance (p < 0.05). Peak impact force, average rate of loading, and peak rate of loading of impact force were all lower when the orthotic device was used (p < 0.05). The findings of this study highlight the potential of a commercially available orthotic to provide benefits more typically associated with molded prescription orthoses, providing a cost-effective option to the routine use of prescription orthotic devices.