| ubox | Universe Sandbox Simulation Data |
| ubs | Microsoft BugsLayer Data |
| ubu | UEFI BIOS Updater Data |
| ucc | National UCC Fininacing Statement Data |
| uccapilog | Windows Live Messenger UCC API Log |
| ucd | AVS Unstructured Cell Data |
| uce | Microsoft Windows Unicode Extensions Data |
| ucf | Xilinx User Constraints File |
| uci | UFS Explorer Compressed Image |
| ucn | Unisys U-ACT Configurator Data |
| ucr | MicroSim PCBoard Unrouted Nets Report Data |
| ucs | F5 User Configuration Set |
| uct | UC Browser Theme |
| ucv | Universal Converter Conversion Data |
| ud | OmniPage User Dictionary |
| uda | Zemax User-defined Aperture File |
| udb | Microsoft Dynamics AX User Database |
| udc | Adobe InDesign User Dictionary |
| uddf | Universal Dive Data Format Data |
| udf | Windows NT Uniqueness Database File |
| udg | uDraw Graph Data |
| udi | PANalytical Peak Data |
| udid | Android Unique Device Identifier Data |
| udl | MDAC Universal Data Link |
| udp | Corel VideoStudio Scene Information |
| uds | Generations Family Tree Document |
| udstate | Motorola Media Finder Data |
| udt | Visilogic Utilities Data Table |
| udt | wyUpdate Update Details File |
| udx | Philips Xenium Mobile Contacts |
| udz | UDA ConstructionSuite Data |
| ue | Uuencoded Data |
| uef | Unified Emulator Format |
| uel | Tekla Structures Custom Components Data |
| uenc | UltraEdit Encrypted Text |