| Abbreviation | Definition |
|---|---|
| UBSS | University of Bristol Spelæological Society |
| UCET | United Cavers Exploration Team |
| UE | Urban Exploration |
| UGHS | Utah Ghost Hunters Society |
| UHT | Universal Home Table |
| UIS | Union Internationale de Speleologie |
| UKC | United Kingdom Coinage |
| UKSC | UK Show Carz |
| UMCCA | United Maine Coon Cat Association |
| UMK | Uncle Mike's injection-molded Kydex Holsters |
| UNW | United Northwest Model Railroad Club |
| UR | Ultra Rare |
| URA | Unió de Radioaficionats Andorrans |
| URG | United Rookie Game Programmers |
| URL | Underground Racing League |
| URPC | Ukiah Rifle and Pistol Club |
| URPC | United Rizal Pigeon Club |
| USABDA | United States Amateur Ballroom Dancers Association |
| USCA | United SideCar Association |
| USCCS | United States Car Club of Switzerland |