Stair walking is frequently encountered in daily living. This task is biomechanically challenging, and can be dangerous for individuals with impaired muscle function. Individuals with unilateral transtibial (below-knee) amputation (TTA) do not have functional use of the ankle plantarflexor muscles and often experience dynamic instability during walking on level ground. TTA also have altered kinematics and kinetics during stair walking compared to able-bodied individuals (AB), which may contribute to increased fall risk. Whole-body angular momentum must be regulated in order to maintain balance during walking. Previous studies have investigated the effects of amputation on whole-body angular momentum during walking on level, inclined and declined surfaces, but no study has yet evaluated whole-body angular momentum in TTA during stair ascent and descent. Also, powered prostheses have recently been developed and have been shown to reduce metabolic costs and work performed by the intact limb during level walking, but it is not known how a powered prosthesis affects the ability to maintain dynamic balance during stair walking. The purpose of this study was to evaluate whole-body angular momentum during stair ascent and descent in TTA compared to AB, as well as to investigate the effects of using a powered prosthesis compared to a passive prosthesis. Ground reaction forces, external moment arms and joint powers were used to interpret observed differences in whole-body angular momentum. Significant differences between walking conditions were found for TTA wearing both types of prosthesis and AB. In general, the range of angular momentum was reduced during stair descent relative to level walking and stair ascent, which may be a mechanism to reduce the risk of falls during stair descent. Significant differences were also found in the range of sagittal whole-body angular momentum during stair ascent for TTA using both types of prostheses compared to AB, but no significant differences in whole-body angular momentum were observed between the passive and powered prosthesis. Differences were also observed in ground reaction forces, external moment arms, and joint powers between TTA using either type of prosthesis and AB, but there were few differences between the passive and powered prosthesis. These results indicate that TTA must compensate for impaired muscle function during stair ascent and descent, and that use of a powered prosthesis does not significantly affect the ability to regulate whole-body angular momentum compared to a passive prosthesis.