Most prosthetic feet behave like springs, and their stiffness affects many important facets of amputee gait. Despite the importance of prosthesis stiffness, the ability of amputees to sense stiffness changes-that is, distinguish between more or less stiff feet-is unknown. This perceptual resolution has implications for the methodology and overall significance of selecting the optimal foot stiffness during prescription. In this experiment, we used a custom, variable-stiffness ankle prosthesis to make small adjustments to stiffness in between steps, and below-knee amputees were asked to identify whether the ankle became more or less stiff. We determined that the average difference threshold of stiffness was 8%, meaning that subjects could correctly identify an 8% change in stiffness 75% of the time. This high sensitivity underscores the importance of optimizing prosthesis stiffness on an individual basis, and suggests a shift is needed in the characterization of commercial feet and the use of stiffness variation during the prescription process.