A passive self-adjusting seat assembly for a lawn maintenance vehicle having a frame. The passive self-adjusting seat assembly includes a base, a guide assembly attached to the base, and a platform assembly that is selectively movable or slidable relative to the guide assembly in order to maintain an operator in an upright or substantially vertical orientation as the lawn maintenance vehicle traverses uneven or sloped ground. The seat assembly further including a locking assembly for selectively locking the sliding or movement of the platform assembly relative to the guide assembly.