A seat slide position detection device for a vehicle includes a lower rail attached to a vehicle floor. The lower rail has one side and another side opposite to the one side in the width direction. The one side includes a side surface and an upward wall portion extending upward along the side surface. An upper rail is supported by the lower rail and slidable with respect to the lower rail in the longitudinal direction of the lower rail. A detection target portion is provided in the upward wall portion of the lower rail in a predetermined range in the longitudinal direction of the lower rail. A position sensor is disposed at the upper rail between the upper rail and the upward wall portion to face the detection target portion.