A mounting flange for attaching an HVAC air diffuser terminal in an opening in a floor or other mounting surface. The mounting flange includes a trim ring and a flange clip. The flange clip is adjustably and movably received in a slot defined a side of the trim ring. A pawl on the flange clip engages with ratchets defined in a surface of the trim ring adjacent to the slot to maintain the flange clip in selected position relative the underside of a floor. The air diffuser terminal is secured in the floor opening between gripping surfaces on the flange clip and the trim ring. An optional screw operably connects the flange clip with the trim ring so that the diffuser may be securely tightened in the floor opening.