The anatomical ankle is capable of providing adaptation to sloped surfaces, a function that is not available in most traditional lower limb prostheses. Commercial prostheses that are claimed to adapt to slopes are limited by high cost, delay in response, reduced stability, and loss of energy through damping. The purpose of the present work was to develop a prototypical prosthetic ankle unit that adapts to sloped surfaces and is sufficiently durable for short-term take-home trials. The prototype switches between low and high rotational impedances by means of a wrap spring clutch mechanism. The clutch is held in a disengaged position when unloaded and deflection of a compressible pylon under axial load rotates a control collar and engages the clutch. The prototype was subjected to 100,000 cycles of mechanical endurance testing based on ISO 10328 standards to determine the suitability for two-week take-home testing. Three persons with unilateral transtibial amputations were recruited to test the prototype in the laboratory, providing subjective feedback through a survey and participating in a motion analysis study to confirm the performance of the slope adaptation function. Translation of the ankle moment-angle curves for all subjects along the ankle angle axis demonstrated a change of the ankle alignment when subjects walked with the adaptable ankle on surfaces of different slopes. The ankle moment-angle curves had a lower slope than the subjects’ usual prostheses, and some subjects had distinct flat regions in the moment-angle curves when using the adaptable ankle. The arbors of the clutch demonstrated significant wear when tested to 100,000 cycles based on ISO 10328 standards, yet the adaptable ankle continued to hold testing loads. The alignment change observed for sloped surfaces suggests the prototype was providing slope adaptation. The flat regions on the ankle moment-angle curves suggest the clutch may have been slipping. Refinement of the clutch engagement mechanism and continued development to reduce the weight and size of the prototype is needed prior to take-home testing.