A vehicle seat rail assembly has a lower rail having a guide groove opening upward in the vertical direction of the vehicle body, that extends in a direction coincident with the longitudinal direction of the vehicle, and supported stationary relative to the vehicle body; an upper rail attached to a vehicle seat and guided by the lower rail to slide in the longitudinal direction of the vehicle; and a cover mechanism that is disposed between a pair of inner walls that define the guide groove of the lower rail and that closes the guide groove. The cover mechanism includes a stationary cover that remains stationary relative to the lower rail and a movable cover that slides relative to stationary cover.