A conceptual design has been generated for a prosthetic ankle-foot mechanism that can automatically adapt to the slope of the walking surface. To help prove this concept, a prototype ankle-foot mechanism was designed, developed, and tested on three subjects with unilateral transtibial amputations walking on level and ramped surfaces. The mechanism is capable of automatically adapting to the walking surface by switching impedance modes at key points of the gait cycle. The mechanism simulates the behavior of the physiologic foot and ankle complex by having a low impedance in the early stance phase and then switching to a higher impedance once foot-flat is reached. The “set-point” at which these changes in impedance occur gets reset on every step in order to reach the proper alignment for the walking surface. The mechanism utilizes the user’s bodyweight to help switch impedance modes and does not require any active control. It was hypothesized that the ankle-foot mechanism would cause the equilibrium point of the ankle moment versus the ankle dorsiflexion angle curves to shift to accommodate the walking surface. For two of the three subjects tested, this behavior was confirmed, supporting the contention that the design provides automatic adaptation for different walking slopes. Further work is needed to develop the prototype into a commercial product, but the mechanism was sufficient for illustrating proof-of-concept.