HBCU National Security Innovation Act of 2021 This bill authorizes the Office of the Under Secretary of Defense for Research and Engineering within the Department of Defense (DOD) to establish activities to better connect historically Black colleges and universities (HBCUs) and minority-serving institutions to the programs of the Defense Innovation Unit (DIU). DOD must brief the congressional defense committees on activities conducted to expand DIU programs to HBCUs and minority-serving institutions and recommendations for how DOD and the federal government can support such institutions to successfully participate in DIU programs.