| U |
| U | Unclassified |
| U.M.D.C. | Mobile Units for the Defense of Christendom (French) |
| UAL | Unit Allowance/Authorization List |
| UAR | United Arab Republic |
| UB | Utility Boat |
| UBA | United Buddhist Association |
| UBI | United Buddhist Institute |
| UCMJ | Uniform Code of Military Justice |
| UD | Infrared Intrusion Detectors |
| UDT | Underwater Demolition Team |
| UE | Unit Equipment |
| UHF | Ultrahigh Frequency |
| UHT | Undergraduate Helicopter Training |
| UHT | Unit Horizontal Tail (applied to F-8 aircraft) - a tail design whereby the whole surface rotates about a pivot point. |
| UI | Unidentified |
| UK | United Kingdom |
| UKEMB | United Kingdom Embassy |
| UMD | Unit Management Document |
| UMNO | United Malay National Organization |
| UNC | United Nations Command |
| UNGA | United Nations General Assembly |
| UNICEF | United Nations International Children's Emergency Fund |
| Unit (of turn) | Divisions on an angle-of-attack indicator on F-4 aircraft. |
| Unk/UNK | Unknown |
| Unloading | Decreasing g's |
| UnLoc | Unknown Location |
| UNS | Unserviceable |
| UPI | United Press International |
| UPT | Undergraduate Pilot Training |
| US | United States |
| US AMB | United States Ambassador |
| US-FWMAF | US-Free World Military Assistance Forces |
| USA | United States Army |
| USAAD | United States Army Advisory Detachment |
| USAAG | United States Army Advisory Group |
| USABVAPAC | United States Army Broadcasting and Visual Activity, Pacific |
| USACAV | United States Army Construction Agency, Vietnam |
| USACDC | United States Army Combat Development Command |
| USAECAV | United States Army Engineer Construction Agency, Vietnam |
| USAECV | United States Army Engineer Command, Vietnam |
| USAF | United States Air Force |
| USAHAC | United States Army Headquarters Area Command |
| USAHSVCGPV | United States Army Health Services Group, Vietnam |
| USAID | United States Agency for International Development (United States Operations Mission, USOM, prior to 1966). |
| USAILC | United States Army International Logistical Center |
| USAIRA | United States Air Attache |
| USALCJ | United States Army Logistical Command, Japan |
| USAMC | United States Army Materiel Command |
| USAMEDCOMV | United States Army Medical Command, Vietnam |
| USAPAV | United States Army Procurement Agency, Vietnam |
| USAPDAV | United States Army Property Disposal Agency, Vietnam |
| USAREUR | United States Army Europe |
| USARHAW | United States Army Hawaii |
| USARJ | United States Army Japan |
| USARMA | United States Army Attache |
| USARMYF | United States Army Forces |
| USARPAC | United States Army Pacific |
| USARPAV | United States Army Procurement Agency, Vietnam |
| USARV | United States Army, Vietnam |
| USARV/MACV SUPCOM | United States Army Vietnam/Military Assistance Command, Vietnam Support Command |
| USARYIS | Unites States Army Ryukyus Island |
| USASCC | United States Army Strategic Communications Command |
| USASCV | United States Army Support Command, Vietnam |
| USASF | United States Army Special Forces |
| USASFG | United States Army Special Forces Group |
| USASFGV | United States Army Special Forces Group in Vietnam |
| USASFV(P) | United States Army Special Forces, Vietnam (Provisional) |
| USASMC | United States Army Supply and Maintenance Command |
| USASPAC | United States Army Security Agency, Pacific |
| USASTRATCOM | United States Army Strategic Communications Command |
| USASWS | United States Army Special Warfare School |
| USATFA | United States Army, Task Force Alpha |
| USC | United States Code |
| USCDA | United States Army Catalog Data Agency |
| USCG | United States Coast Guard |
| USCGC | United States Coast Guard Cutter |
| USD | Unattended Seismic Detector |
| USDAO | United States Defense Attache Office |
| USDEL | United States Delegation |
| USDO | United States Disbursing Officer |
| USEF | Unidentified Size Enemy Force |
| USEMB | United States Embassy |
| USFJ | United States Forces, Japan |
| USFK | United States Forces, Korea |
| USG | United States Government |
| USIA | United States Information Agency |
| USIA | United States Intelligence Agency |
| USINFO | United States Information Office |
| USIS | United States Information Service - The overseas missions of the United States Information Agency. |
| USMA | United States Military Academy |
| USMAAGV | United States Military Assistance Advisory Group, Vietnam |
| USMACTHAI | United States Military Assistance Command, Thailand |
| USMACV | United States Military Assistance Command, Vietnam |
| USMC | United States Marine Corps |
| USMWR | United States Mission Weekly Report(s) |
| USN | United States Navy |
| USNAVFORV | United States Naval Forces, Vietnam |
| USNS | United States Navy Ship |
| USO | United Services Organization |
| USOM | United States Operations Mission (preceding January 66) - The designation in Vietnam of the local mission of the Agency for International Development universally abbreviated to USOM in Vietnam. |
| USOM | United States Overseas Mission |
| USOMREP | USOM Representative |
| USPAO | United States Public Affairs Office |
| USPOL | United States Petroleum Oil Lubricants |
| USREP | United States Representative |
| USS | United States Ship |
| USSAG | United States Support Activities Group |
| USSR | Union of Soviet Socialist Republics |
| USTDC | United States Taiwan Defense Command |
| UTB | Utility Boat |
| UTG | Underway Training Group |
| UTT | Utility Tactical Transport |
| UW | Unconventional Warfare |