A rear door device for a vehicle may include a rear door, an upper rail mounted to the rear door in a width direction, an upper slider inserted in the upper rail to support the upper rail, a lower rail mounted to a vehicle body and formed with a curved portion, a lower slider inserted in the lower rail and movable along the lower rail, an upper support structure having one end connected to the upper slider and the other end mounted to the vehicle body for supporting the upper slider, and a connection structure for connecting the lower slider to the upper rail. The rear door device permits the rear door to be opened or closed by sliding it forward or backward in the length direction of the vehicle.