Return Excessive Migrants and Asylees to International Neighbors in Mexico Act of 2021 or the REMAIN in Mexico Act of 2021 This bill requires the Department of Homeland Security (DHS) to implement the Migrant Protection Protocols as outlined in the January 25, 2019, memo titled Policy Guidance for Implementation of the Migrant Protection Protocols . (The protocols generally required aliens who are not clearly admissible, including those seeking asylum, arriving by land along the U.S.-Mexico border to be returned to Mexico while their immigration proceedings are pending, rather than remain in the United States. On January 21, 2021, DHS stopped applying the protocols to newly-arrived individuals.)