An adapter plate assembly which provides a convenient interface between a mounting support assembly for suspending an instrument driver robotic instrument system above a operating table. The adapter plate securely mounts to the operating table, and may be adjustable so that it can be adjusted to fit various sizes and models of operating tables. The adapter plate assembly comprises a main plate which rests on the surface of the operating table. A first clamp assembly which removably attaches to the operating table is mounted on one or more horizontal adjustment rods which are slidably received in one of the main plate in order to provide adjustability to the width of the adapter plate assembly. On the opposing end of the main plate, a second clamp assembly is coupled main plate. In order to secure the support assembly of a robotic instrument system, an adapter rail is also coupled to the main plate. The adapter rail is configured to detachably couple to a support assembly interface of the support assembly, and provides sufficient structural support for suspending the instrument driver above the operating table.