A novel wheelchair moving platform with varying wheelbase and tread is designed and built. The width of wheelchairs for residential use is limited to the width of standard doorways which the wheelchair must go through. Yet the wheelchair must have an enough stability margin although the patient’s mass centroid may significantly change. In this paper, a new mechanism for varying the footprint of a four-wheeled omnidirectional wheelchair is presented. The chair is mounted on a moving platform with four ball wheels allowing for holonomic, omnidirectional motion. Two pairs of ball wheels at the diagonal positions of the chassis are fixed, respectively, to two beams that intersect at the pivotal point in the middle. The angle between the two beams crossing at the pivotal point is varied actively so that the ratio of the wheelbase to the tread can be varied. Four independent servomotors driving the four ball wheels allow the wheelchair to move along three axes, i.e. x, y, and ϕ, and change the footprint at the same time. The footprint configuration is controlled based on the measurement of the mass centroid position in such a way that the mass centroid can be kept within the footprint at all times. First the concept of the reconfigurable footprint mechanism is described, and its kinematic and static behavior is analyzed, followed by the control design for augmenting the vehicle stability. A prototype wheelchair platform is designed, built, and tested.