Hip exoskeletons may hold potential to augment walking performance and mobility in individuals with disabilities. The purpose of this study was to design and validate a novel autonomous hip exoskeleton with a user-adaptive control strategy capable of reducing the energy cost of level and incline walking in individuals with and without walking impairment. First, in a small cohort of three unimpaired individuals, we validated the ability of our control strategy to provide hip flexion-extension torque that was proportional to the biological hip moment and reduce the energy cost of level and incline walking (24 ± 5% and 13 ± 5% reductions, respectively). Next, in a clinical feasibility experiment with an individual with significant walking impairment from cerebral palsy, we demonstrated that our untethered device and adaptive control scheme improved hip extension by 14° across the gait cycle, reduced average rectus femoris and semitendinosus muscle activity by 23% and 46%, respectively, and resulted in a 15% improvement in metabolic cost relative to walking without wearing the device.