A system for mounting a sink to the under-side of a countertop, the sink having a bowl and a laterally extending flange emanating from the bowl, comprising: a pair of rigid side mounting rails adapted to engage the underside of the flange and adapted to engage a side wall of a cabinet on which the countertop is mounted; the side mounted rails being “L” shaped and each having a plurality of openings spaced along the length of the side mounting rails, the openings being adapted to engage a plurality of fasteners and a plurality of threaded bolts; a rigid front mounting rail adapted to engage the underside of the flange and adapted to engaged the front wall of a cabinet on which the countertop is mounted; the front mounted rail being “L” shaped and having a plurality of openings spaced along the length of the side mounting rails, the openings being adapted to engage a plurality of fasteners and a plurality of threaded bolts; the plurality of threaded bolts is adapted to engage the plurality of openings in the side mounting rails and the front mounting rail and adapted to engage a plurality of mounting cups which are operatively associated with a plurality of mounting devices selected from the group comprising: mounting cups, mounting clips, or a combination thereof; and the plurality of mounting devices is adapted to engage the threaded bolts and adapted to engage the underside of the flange of the sink.