| Country | Country name |
| Trinidad and Tobago | conventional long form: Republic of Trinidad and Tobago conventional short form: Trinidad and Tobago |
| Tunisia | conventional long form: Tunisian Republic conventional short form: Tunisia local long form: Al Jumhuriyah at Tunisiyah local short form: Tunis |
| Turkey | conventional long form: Republic of Turkey conventional short form: Turkey local long form: Turkiye Cumhuriyeti local short form: Turkiye |
| Turkmenistan | conventional long form: none conventional short form: Turkmenistan local long form: none local short form: Turkmenistan former: Turkmen Soviet Socialist Republic |
| Turks and Caicos Islands | conventional long form: none conventional short form: Turks and Caicos Islands abbreviation: TCI |
| Tuvalu | conventional long form: none conventional short form: Tuvalu local long form: none local short form: Tuvalu former: Ellice Islands note: "Tuvalu" means "group of eight" referring to the country's eight traditionally inhabited islands |
| Uganda | conventional long form: Republic of Uganda conventional short form: Uganda |
| Ukraine | conventional long form: none conventional short form: Ukraine local long form: none local short form: Ukrayina former: Ukrainian National Republic, Ukrainian State, Ukrainian Soviet Socialist Republic |
| United Arab Emirates | conventional long form: United Arab Emirates conventional short form: none local long form: Al Imarat al Arabiyah al Muttahidah local short form: none former: Trucial Oman, Trucial States abbreviation: UAE |
| United Kingdom | conventional long form: United Kingdom of Great Britain and Northern Ireland; note - Great Britain includes England, Scotland, and Wales conventional short form: United Kingdom abbreviation: UK |
| United States | conventional long form: United States of America conventional short form: United States abbreviation: US or USA |
| United States Pacific Island Wildlife Refuges | conventional long form: none conventional short form: Baker Island; Howland Island; Jarvis Island; Johnston Atoll; Kingman Reef; Midway Islands; Palmyra Atoll |