Territories Medicare Prescription Drug Assistance Equality Act of 2021 This bill makes certain income-based subsidies under the Medicare prescription drug benefit available to individuals who reside in Puerto Rico or another U.S. territory and are otherwise eligible. Current law makes such individuals ineligible for premium and cost-sharing subsidies, but establishes a process for U.S. territories to apply for financial assistance with respect to the provision of Medicare prescription drugs. The bill retains a process for U.S. territories to apply for such assistance, but alters the formula by which the amount of assistance is calculated. Specifically, the bill phases in modifications to the formula that more closely align how assistance is calculated for U.S. territories with how it is calculated for the 50 states and the District of Columbia.