Transatlantic Telecommunications Security Act This bill directs the Department of State and other federal agencies to help Central and Eastern European countries improve the security of their telecommunications networks. A project located in a Central or Eastern European country shall be eligible for assistance if it (1) improves telecommunications networks; (2) is inclusive, transparent, economically viable, sustainable, supplied only with secure equipment and services, and compliant with international standards and laws; (3) does not use equipment or services from China or replaces such equipment or services; and (4) enhances telecommunications market integration across the Central or Eastern European region that is secure against malign actors. To support eligible projects, the State Department shall provide diplomatic and political support and shall also encourage international financial institutions to provide investment support. The U.S. Trade and Development Agency, the U.S. International Development Finance Corporation, and other specified federal agencies shall also provide project support, such as feasibility studies and finance-related support, as necessary.