Arm Taiwan Act of 2021 This bill requires the Department of Defense (DOD) to establish the Taiwan Security Assistance Initiative to accelerate Taiwan's deployment of asymmetric defense capabilities to the level required to deter or defeat an invasion by China. From FY2023 through FY2027, the bill requires DOD to use appropriations provided for the initiative to provide assistance to accelerate Taiwan's asymmetric defense capabilities so that Taiwan, with limited initial U.S. Armed Forces support, is able to (1) delay, degrade, and deny attempts by China's army to enter or transit the Taiwan Strait (and adjoining seas) or seize control of key territory in Taiwan; and (2) prevent China from neutralizing or rendering ineffective Taiwan's government. The bill imposes certain requirements related to such assistance, such as periodic certifications by DOD to Congress that Taiwan's government has committed to (and has made progress toward) spending at least 3% of Taiwan's national gross domestic product on defense on an annual basis by the end of FY2027.