| Extension | Information |
| U | Subsampled Raw YUV Bitmap |
| U | Unreal System File |
| U | Unreal Tournament 2003 System File (Epic Games, Inc.) |
| U | Nitpicker Ipflow File |
| U2 | Telelogic Tau UML Model |
| U2E | Unreal Game Translation |
| U2P | Telelogic Tau UML Program Text File |
| U3D | Universal 3D File |
| U3P | U3 Platform Install File (U3 LLC.) |
| U4A | UnInstaller Archive |
| U8 | Raw Unsigned 8-Bit Audio Data |
| U96 | EasyZip Temporary File |
| U98 | Urdu 98 Document |
| UAE | WinUAE 0.8.22 Game Configuration File |
| UAP | User Agent Wireless Telephony Profile |
| UAX | Unreal Audio File (Epic Games, Inc.) |
| UAX | Unreal Tournament 2003 Sound (Epic Games, Inc.) |
| UAX | Deus Ex |
| UAX | Harry Potter |
| UAX | Rune |
| UAX | Undying |
| UAX | Unreal / Unreal 2 |
| UAX | Unified Application eXchange Format (UAX Project) |
| UB | Audio |
| UB | Raw Unsigned Byte (8-bit) Data |
| UBB | BASIC UBAS File |
| UBD | BASIC UBAS File |
| UBK | National Construction Estimator 32 User Costbook |
| UBK | Embiid Reader eBook (Embiid Inc.) |
| UBM | Iphoto Mtwain Install File |
| UBS | Tracesrv Bugslayer Utility File |
| UC | Unreal Tournament Code File (Epic Games, Inc.) |
| UC | AutoCut Flash Firmware File (Autometrix) |
| UC2 | UltraCompressor II Compressed Archive (AIPNL) |
| UCD | Prosa/ OM Use Case Diagram |
| UCE | UniCode Extensions |
| UCF | Xilinx Foundation User Constraint File |
| UCG | ULEAD MediaStudio CG Infinity Graphic (Ulead Systems, Inc.) |
| UCM | Urban Chaos Game File |
| UCN | Compressed Archive |
| UCN | U-Act Configurator Configuration Storage File (Unisys) |
| UCR | MicroSim PCBoard Unrouted Nets Report |
| UCS | Universal Classification Standard Database File |
| UCX | Rational XDE (IBM) |
| UD | AOL 4.0 (America Online, Inc.) |
| UD | OmniPage User Dictionary (ScanSoft, Inc.) |
| UDB | Works File (Microsoft) |
| UDB | PINO 3-Server File |
| UDB | Windows2000 Uniqueness Database File (Microsoft) |
| UDC | Acrobat Spelling File (Adobe) |
| UDC | Fh5 US English File |
| UDD | OllyDbg Module Information (Oleh Yuschuk) |
| UDF | Excel User Defined Function (Microsoft) |
| UDF | Photostyler Image Filter |
| UDF | Windows NT Uniqueness Database File |
| UDI | Corel Graphics Ver 10 Draw Script Textures Bitmap File (Corel) |
| UDL | Data Link (Microsoft) |
| UDM | MapView User-Defined Module (MotCreations) |
| UDS | Sierra Generations Geneology Family File (Sierra Entertainment, Inc.) |
| UDS | FIMware Database (Uniform Data System for Medical Rehabilitation, a division of UB Foundation Activities, Inc.) |
| UDW | Raw Unsigned DoubleWord (32-bit) Data |
| UE2 | Encrypted Archive |
| UEF | Unified Emulator Format |
| UEZ | COOL 3D (Ulead Systems, Inc.) |
| UFA | UFA Compressed File Archive |
| UFA | GIF Animator Animation Object Information (Ulead) |
| UFF | DataWave Physiological Data File |
| UFO | PhotoImpact Graphic (Ulead Systems, Inc.) |
| UFO | Total Annihilation Third-party Unit (Cavedog Entertainment) |
| UG | Drawing File |
| UGA | Ulead GIF Animator Project File (Ulead) |
| UH | Age of Kings (Microsoft) |
| UHA | UHARC Compressed Archive |
| UHS | Universal Hint System File (Universal Hint System) |
| UHTM | Unreal Tournament File |
| UI | Geoworks UI Compiler Espire Source |
| UI | User Interface |
| UID | IBM Voice Type Users File (IBM) |
| UIF | WordPerfect Long Prompts (Corel Corporation) |
| UIH | Geoworks UI Compiler Espire Header |
| UIL | X-Motif UIL Table |
| UIN | ICQ 2001+ Saved ICQ Contact Information (ICQ Inc.) |
| UIS | WindowBlinds Skin |
| UKA | Fifa World Cup Game Data fe Art Legalscr File |
| UKA | UKA Data Archive (HRM Systems AG, CH-Winterthur) |
| UKS | Works File (Microsoft) |
| UKX | Unreal Tournament 2003 Animation (Epic Games, Inc.) |
| UK_ | Mcguiness File |
| UL | uLAW Audio File |
| ULD | Procomm Uploaded Files |
| ULF | Uniware Layout File (Uniware) |
| ULP | Peugeot Diag 2000 File |
| ULP | EAGLE Layout Editor Script (CadSoft Computer GmbH) |
| ULS | Internet Location Service |
| ULT | UltraTracker Music Module |
| ULW | ULAW - Sun/NeXT |
| ULX | Glulx Game |
| ULX | CLAN Lexical File (Child Language Data Exchange System) |
| UM | The Land Of UM World File |
| UMB | MemMaker Backup Archive |
| UMI | CDDB Database and PC Music Library in PlayCenter |
| UML | Visual UML Class Diagram & Model (Visual Object Modelers, Inc.) |
| UMOD | Unreal Self-extracting MOD File (Epic Games) |
| UMOD | Unreal Tournament (Epic Games, Inc.) |
| UMS | Rune |
| UMX | Unreal/Unreal Tournament Music File (Epic Games, Inc.) |
| UMX | Unreal Tournament 2003 Music (Epic Games, Inc.) |
| UMX | Deus Ex |
| UMX | Harry Potter |
| UMX | Unreal |
| UND | A86 Assembler Undefined Symbols |
| UNF | Btreive Unformatted File |
| UNI | Forcast Pro Data File |
| UNI | MikMod (UniMod) Format |
| UNIX | UNIX File |
| UNL | Informix Binary File Created by UNLOAD Command |
| UNM | XMap Flight Plan or Flight Plan Statistic File (Delorme) |
| UNQ | ACT! Saved Unknown Attachment (Best Software CRM Division) |
| UNQ | Fax View File |
| UNR | Unreal Map File |
| UNT | AutoCAD Unit Definition (Autodesk, Inc.) |
| UNUSED | Photoline4 Defaults File |
| UNV | BusinessObjects/WebIntelligence Universe File |
| UNV | SDRC I-DEAS File |
| UNV | Forecast Pro Univariate Forecast File (Business Forecast Systems, Inc.) |
| UNX | Text File |
| UN_ | Ntgraph Setup30 file |
| UOL | PhotoImpact Object Library File |
| UP2 | Cool3d Plug-ins Board File |
| UPC | Ultimate Paint (J-T-L Development Inc.) |
| UPD | dBASE Update File |
| UPD | ProUpdater Per/Directory Options Setting File |
| UPD | Universal Print Driver |
| UPD | Updated Program Data |
| UPD | VideoStudio Scene Information (Ulead Systems, Inc.) |
| UPDATE | ISAPI Loader New ISAPI Application (EGGcentric) |
| UPDATE | Program or Data File Update |
| UPDF | UltraPdf Publication (Software995) |
| UPF | Nokia Phone Digital Photo (Nokia) |
| UPF | Universal Picture Format Bitmap |
| UPG | Grafik Anim File |
| UPG | Firmware Upgrade File |
| UPI | Iphoto4 Programs Plug-in File |
| UPI | ULead Photo Impact Program File |
| UPJ | ULead Project File |
| UPL | Starmoney Agent Download File |
| UPM | PenMail Document (UC-Logic Technology Corp.) |
| UPO | dBASE Compiled Update File |
| UPR | FileMaker User Spelling Dictionary |
| UPS | Works File (Microsoft) |
| UPT | Connectivity Memory Model Update Timing Input File |
| UPX | ULead Photo Express Saved Image File |
| UPZ | Starmoney Agent Download File |
| URF | ProWORX Nxt User Configurable Register Editor File (Schneider Electric) |
| URF | Radar ViewPoint Universal Radar Format |
| URF | AppFace Skin (Matinsoft Inc.) |
| URI | List of Uniform Resource Identifiers |
| URIS | List of Uniform Resource Identifiers |
| URL | Internet Location |
| URLS | GetRight URL List (Headlight Software, Inc.) |
| URS | ControlDraw User Requirements Specifier (ControlDraw Ltd.) |
| URT | Utah Raster Image |
| USA | Office Header File (Microsoft) |
| USA | Unreal Saved Game File |
| USAGE | Snort File |
| USB | D-Link FM Radio Update |
| USB | Syssoft Sandra File |
| USB | USB Protocol Analyzer Trace |
| USE | Log File |
| USE | MKS Source Integrity File |
| USERPROFILE | Live Update User Profile (Symantec) |
| USF | Core Media Player XML Universal Subtitle Format File |
| USF | Nintendo Ultra 64 Sound Format |
| USI | UsiXML UI Description File (Universite Catholique de Louvain-la-Neuve) |
| USL | LaserJet Landscape Font |
| USL | ProWORX Nxt USL Loadables (Schneider Electric) |
| USL | PC-Bibliothek User Dictionary (Bibliographisches Institut & F.A. Brockhaus AG) |
| USP | LaserJet Portrait Font |
| USP | PageMaker Printer Font (Adobe) |
| USP | SIMCA-P/SIMCA-P+ Project (Umetrics AB) |
| USR | User Database |
| USR | ERGO Receiver Configuration File (Creative Express Corporation) |
| USR | MicroImages User-specific Information (MicroImages, Inc.) |
| USR | Uplink Player Information File (Introversion Software) |
| USR | Photo Paint User Defined Filter (Corel) |
| USR | Pegasus Mail User Information (David Harris) |
| USR | FileMaker Pro Runtime Engine Database Application (FileMaker, Inc.) |
| USS | PhotoImpact Exported Slicing Pattern (Ulead Systems, Inc.) |
| USS | WinUAE Saved State |
| USTAR | POSIX tar Compressed Archive |
| USX | Unreal Tournament 2003 Static Mesh (Epic Games, Inc.) |
| USZ | UsiXML UI Description File (Universite Catholique de Louvain-la-Neuve) |
| UT! | µTorrent Incomplete Download (Ludvig Strigeus) |
| UT2 | Unreal Tournament Map (Epic Games, Inc.) |
| UT2MOD | Unreal Tournament 2003 Game Modification File (Epic Games) |
| UT4MOD | Unreal Tournament 2004 MOD Installation Package (Epic Games) |
| UTE | Rukus 110 File |
| UTF | AOL Updating Files (America Online, Inc.) |
| UTI | Spybot - Search & Destroy Usage Tracking (PepiMK Software) |
| UTL | QuickBooks Data |
| UTL | Sound File |
| UTP | BinPatch Patch Data File |
| UTS | Spybot - Search & Destroy Include File (PepiMK Software) |
| UTX | Unicode File |
| UTX | Unreal Tournament 2003 Texture (Epic Games, Inc.) |
| UTX | Deus Ex |
| UTX | Harry Potter |
| UTX | Rune |
| UTX | Undying |
| UTX | Unreal / Unreal 2 |
| UU | Compressed Archive File |
| UUD | Uudecoded |
| UUE | Uuencoded |
| UUT | Genesis 2000 (Genesis 2000, Inc.) |
| UV | EA Games Video File (Electronic Arts, Inc.) |
| UV2 | Jane's Combat Simulations F-15 |
| UVP | ULEAD MediaStudio Video Paint Graphic (Ulead Systems, Inc.) |
| UVR | Ulead Cool 360 Viewer File |
| UVV | Listlabl File |
| UVW | geoManager/geoInterface User View |
| UVX | Unreal Tournament 2003 Saved Game (Epic Games, Inc.) |
| UW | Audio |
| UW | Raw Unsigned Word (16-bit) Data |
| UW | Unitype GlobalWriter File |
| UWF | UltraTracker Wave File |
| UWL | WordPerfect User Word List (Corel Corporation) |
| UWS | UniTrader 2006 Website Archive (UniTechnology Limited) |
| UX | UNIX File |
| UXD | URLBase URL Database (Terria Development) |
| UXP | URLBase URL Database (v. 5.1) (Terria Development) |
| UXX | Unreal Tournament 2003 Cache (Epic Games, Inc.) |
| UXY | Orchida Embroidery System (Orchida Soft) |
| UYVY | Imagemagick-5.3.2 Perlmagick File |
| UYVY | Interleaved YUV Raw Image |
| UZ | Unreal Tournament Server File |
| UZE | Ultimate Zip Compresion Agent |
| UZR | UZR 3D Streamable 3D File Format |