Abstract: The HVAC register boot-mounting system is a register boot comprising a left clamp and a right clamp. The left and right clamps may be moved to a first position where they do not extend outside of the envelope of the boot so that the boot may be installed into a hole. A plurality of flanges limit the depth of the insertion. The left and right clamps may then be pivoted to a second position where each clamp extends outside of the boot, clamping drywall around the hole between the plurality of flanges and the clamps. An HVAC duct may be coupled to a duct connector on the top of the boot and a vent may be inserted into an aperture on the bottom of the boot. The presence of the vent prevents the clamps from pivoting back to the first position.