| U | Unionist; University |
| UAE | United Arab Emirates |
| UBC | University of British Columbia |
| UC | University College |
| UCAS | Universities and Colleges Admissions Service |
| UCE | University of Central England |
| UCH | University College Hospital (London) |
| UCL | University College London (University) |
| UCLA | University of California at Los Angeles |
| UCNW | University College of North Wales |
| UCS | University College School |
| UCW | University College of Wales; Union of Communication Workers |
| UDC | Urban District Council; Urban Development Corporation |
| UDR | Ulster Defence Regiment; Union Democrates pour la Veme Republique |
| UE | United Empire Loyalist (Canada) |
| UEA | University of East Anglia |
| UFC | Universities' Funding Council |
| UGC | University Grants Committee (later UFC) |
| UK | United Kingdom of Great Britain and Ireland 1801-1922; United Kingdom of Great Britain and Northern Ireland 1922-; used chiefly of peerage and baronetage creations of |
| UKAEA | United Kingdom Atomic Energy Authority |
| UKLF | United Kingdom Land Forces |
| UMDS | United Medical and Dental Schools |
| UMIST | University of Manchester Institute of Science and Technology |
| UN | United Nations |
| UNA | United Nations Association |
| unc | uncle |
| UNIFEM | United Nations Development Fund for Women |
| Univ | University - College (Oxford) |
| unm | unmarried |
| UNO | United Nations Organization |
| UNRRA | United Nations Relief and Rehabilitation Administration |
| UP | United Provinces - India; Utter Pradesh; United Presbyterian |
| URC | United Reformed Church |
| US(A) | United States (of America) |
| USAAF | United States Army Air Force |
| USAF | United States Air Force |
| USAR | United States Army Reserve |
| USC | University of Southern California |
| USDAW | Union of Shop Distributive and Allied Workers |
| USMC | United States Marine Corps |
| USN | United States Navy |
| USNR | United States Naval Reserve |
| USPG | United Society for the Propagation of the Gospel |
| USS | United States Ship |
| USSR | Union of Soviet Socialist Republics |
| UT | Utah |
| Utd | United |
| UU | Ulster Unionist |
| UWE | University of the West of England |
| UWIST | University of Wales Institute of Science and Technology |