ABSTRACT: Lower limb amputees exhibit a high incidence of falls, and many express a fear of falling. The goal of this research was to develop a powered inverting and everting prosthetic foot and discover if it can improve amputee balance. This article describes our progress toward this goal. A novel prosthetic foot consisting of a four-bar mechanism and a linear actuator attached to the keel of a conventional prosthetic foot enabled coronal plane rotation about the ankle axis. Signals from a pylon-mounted load cell were used to provide feedback control of inversion or eversion movements. A robotic bench test was used to measure system response time. A transfemoral amputee (n = 1) wearing the prosthesis participated in tests with and without the control system turned on to measure effects on standing balance and gait. Movement of the center of pressure (sway) while standing with the feet together on a force plate with and without visual feedback was used to quantify effects on standing balance. Step width before and after medially and laterally directed disturbances was used to quantify effects on walking balance. The robotic bench test showed that the settling time in response to a 2-degree step input was ∼180 milliseconds. The standing balance test showed that the prototype prosthesis had no effect with eyes open but reduced sway by 50% with eyes closed. The prosthesis enabled a quick recovery to predisturbance step widths in response to a laterally directed disturbance but had no effect on medially directed disturbances. The results suggest that this first-generation prototype may improve amputee balance, but further work to improve the response time of the device and reduce its weight and length is warranted before undertaking a clinical trial.