Adoptee Citizenship Act of 2021 This bill provides for the automatic acquisition of citizenship for certain individuals adopted by a U.S. citizen. A child born outside the United States and adopted by a U.S. citizen shall automatically acquire citizenship upon meeting certain requirements, regardless of when the adoption was finalized. Currently, adoptees who were over the age of 18 on February 27, 2001, do not automatically acquire citizenship. An individual born outside the United States and residing in the United States shall automatically acquire citizenship if the individual (1) was adopted by a citizen before becoming 18 years old, (2) was physically present in the United States in the citizen parent's custody pursuant to lawful admission before becoming 18 years old, (3) never acquired citizenship before this bill's enactment, and (4) was lawfully residing in the United States on this bill's enactment date. An individual born outside the United States and residing outside the United States but who otherwise meets all of the requirements shall automatically acquire citizenship upon being physically present in the United States pursuant to lawful admission. Such an individual shall be subject to a background check. If the background check reveals the individual has committed a crime that was not properly resolved, the individual may not receive a visa unless the Department of Homeland Security and Department of State have coordinated with law enforcement to ensure that action was taken to resolve the issue.