The anatomical ankle is capable of providing adaptation to sloped surfaces, a function that is unavailable in most traditional lower limb prostheses. Commercially available prostheses that are claimed to adapt to surfaces have limitations such as high cost, delay in response, reduced stability, and loss of energy through damping. The purpose of the present work was to develop a prototype prosthetic ankle that adapts to sloped surfaces and is sufficiently durable for short-term field trials. The prototype switches between low and high rotational stiffnesses by means of a wrap spring clutch, and demonstrates a change of the ankle alignment in the ankle moment-angle curves when subjects walked with the unit on surfaces of different slopes, suggesting the prototype was providing slope adaptation. The arbors of the wrap spring clutch demonstrated significant wear when tested to 100,000 cycles based on ISO 10328 standards, yet the adaptable ankle continued to hold testing loads. Further efforts to reduce the weight and size of the prototype are essential, and continued refinement of the clutch engagement mechanism is recommended.