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OBJECTIVE: Many individuals with cerebral palsy (CP) experience gait deficits resulting in metabolically-inefficient ambulation that is exacerbated by graded walking terrains. The primary goal of this study was to clinically-validate the accuracy and efficacy of adaptive ankle exoskeleton assistance during steady-state incline walking and stair ascent in individuals with CP. Exploratory goals were to assess safety and feasibility of using adaptive ankle exoskeleton assistance in real-world mixed-terrain settings. METHODS: We used a novel battery-powered ankle exoskeleton to provide adaptive ankle plantar-flexor assistance during stance phase. Seven ambulatory individuals with CP completed the study. RESULTS: Adaptive controller accuracy was 85% for incline walking and 81% for stair-stepping relative to the biological ankle moment. Assistance improved energy cost of steady-state incline walking by 14% (p = 0.004) and stair ascent by 21% (p = 0.001) compared to walking without the device. Assistance reduced the muscular demand for the soleus and vastus lateralis during both activities. All participants were able to safely complete the real-world mixed-terrain route, with adaptive ankle assistance resulting in improved outcomes compared to walking with the device providing zero-torque; no group-level differences were found compared to walking without the device, yet individuals with more impairment exhibited a marked improvement. CONCLUSION: Adaptive ankle exoskeleton assistance can improve the energy cost of steady-state incline walking and stair ascent in individuals with CP. SIGNIFICANCE: As the first study to demonstrate safety and performance benefits of ankle assistance on graded terrains in CP, these findings encourage further investigation in free-living settings.