Optic flow (OF) stimulation, which uses virtual reality (VR) technology to present images that induce motor vision different from the actual walking speed during treadmill walking, is expected to be a rehabilitation technique that improves the gait function in stroke patients. This study aimed to investigate the effects of VR treadmill gait training under various OF speeds in stroke patients and healthy controls. Twelve healthy young adults and 10 stroke patients walked on a treadmill with speed set at their comfortable walking speed for 5 min each under four conditions: no OF stimulation and OF stimulations at 100%, 125%, and 75% of the treadmill speed. We evaluated the changes in walking time, the number of steps, and walking rate (steps/min) in a 10-m walking test performed before, immediately after, and 5, 10, and 20 min after treadmill training. A robust facilitating effect on gait function was observed in treadmill training with 125% OF stimulation speed. In stroke patients, the walking time and number of steps were significantly reduced relative to the pre-training state from immediately to 10 min after treadmill training at 125% OF stimulation speed. The walking rate also increased significantly 10 min after training. In healthy controls, the walking time decreased significantly from immediately to 10 min after training at 125% OF stimulation speed compared with the pre-training state. The number of steps also decreased significantly, but only immediately after the training, while the walking rate did not change throughout the experiment. In conclusion, OF stimulation at a speed faster than the actual locomotion speed effectively increases the walking speed, and this effect persisted for approximately 10 min after training in both stroke patients and healthy participants who used different neural strategies to enhance gait function.