Securing America's Pharmaceutical Supply Chain Act This bill allows an executive agency to purchase a drug approved or licensed by the Food and Drug Administration only if the drug is over 50% sourced, manufactured, and assembled in the United States. This requirement may be waived (1) if the drug is not available in sufficient quantity or quality as over 50% sourced, manufactured, and assembled in the United States; or (2) during an emergency. The U.S. Trade Representative (USTR) shall modify U.S. product coverage under all free trade agreements and the World Trade Organization Agreement on Government Procurement to exclude coverage of essential medicines and medical countermeasures. Subsequent to such modifications, the USTR shall make any necessary corresponding modifications of existing waivers and notify the Office of Management and Budget.