A litter evacuation door system includes a door opening defined through a side wall portion of a litter tray, a sliding door guide structure surrounding the door opening, a door slidably engaged in the sliding door guide structure, the door is user selectively movable between a closed and open position, a bag engaging collar extending around an exterior side edge of the door opening and configured for engaging the opening edge of a litter collecting bag, the bag engaging collar further defining a bag collar engaging groove for slidably receiving therein the bag collar plate of a disposable litter collecting bag.