An adjustable support device for under-mounted sanitary fixtures which enables the installation of rimless and rimmed fixtures to a countertop without the use of counter attaching hardware. The system includes mounting brackets for attachment to under-counter support cabinetry and moveable carriages suspended from the mounting brackets by adjustable lift actuators. Length adjustable hanger beams are attached to the opposing moveable carriages to create a support surface for the fixtures. The moveable carriages and attached hanger beams are lowered by the lift actuators to accommodate the insertion of a sanitary fixture and then raised to position the supported, upward facing rim of the fixture to the desired position relative to the countertop.