The aim of this study was to validate the effectiveness of the newly developed intelligent foot orthosis (IFO) at preventing lateral falls. The IFO is a wearable fall prevention system based on using a small magnetorheological brake to control the height of the lateral sole. Experiments were performed to compare the walking motions on a lateral slope under four conditions: without IFO, with IFO current-OFF, with IFO current-ON, and with IFO control-ON. The mediolateral center of gravity and center of pressure horizontal distance (ML COG–COP HD) were measured in three-dimensional motion analysis to represent the change in posture on the frontal plane. To observe the corresponding muscular activity, surface electromyography (EMG) was performed to obtain the mean and peak root-mean-square (RMS) for the tibia anterior (TA) and peroneus longus (PL) in the first half of the stance phase when the IFO applied control. In the results, ML COG-COP HD increased significantly under the “with IFO control-ON” compared to the “without IFO” and “with IFO current-ON” conditions. The mean RMS of the TA was significantly decreased under the “with IFO current-ON” and “with IFO control-ON” conditions compared to the “without IFO” condition. These results demonstrate that the posture moved away from the lateral fall direction primarily due to IFO assistance rather than muscular activity, which would be a consequence of human postural control. Thus, the IFO does appear to help prevent lateral falls.