Unmanned Aircraft Systems Review Act of 2021 This bill requires the Office of the Under Secretary of Defense for Acquisition and Sustainment within the Department of Defense (DOD) to review DOD's system for categorizing unmanned aircraft systems (UAS) as described in Joint Publication 3-30, titled Joint Air Operations . The office must also determine whether modifications should be made in DOD's grouping of unmanned aerial systems into five broad categories. If revisions are needed, the office must consider the effect a revision would have on (1) the future capability and employment needs to support current and emerging warfighting concepts, (2) advanced systems and technologies available in the current commercial marketplace, (3) the rapid fielding of UAS technology, and (4) the integration of UAS into the National Airspace System.