| 401 | UCSX | Union Camp Corp. | added 10/1988; 1/1994-7/2000; eliminated 4/2002 | | NAKINA |
| 402 | UCSX | International Paper Co. | added 4/2002; 7/2005 | | NAKINA |
| 403 | UCSX | Union Camp Corp. | added 10/88; 1/94-7/2000 | | NRHS |
| 404 | UCSX | INTERNATIONAL PAPER CO. [UNION CAMP CORPORATION] | | | PWRC |
| 405 | UCSX | Union Camp Corporation | | | RSRV |
| 406 | UCSX | INTERNATIONAL PAPER COMPANY | | | SCAC4 |
| 407 | UCTG | UNIVERSAL CONCORD TRANSAMERICAN GRO | | | SCAC4 |
| 408 | UCTK | UNITED CLEAN TRUCKS | | | SCAC4 |
| 409 | UCTN | UNION CITY TRANS INC | | | SCAC4 |
| 410 | UCUI | U CAN UNIVERSAL INC | | | SCAC4 |
| 411 | UCUY | UNIQUE CUSTOM UPHOLSTERY | | | SCAC4 |
| 412 | UCV | United Company for Aviation Administration | | SA | AVCODES |
| 413 | UCWS | US CONTAINER AND WARCHOVING SERV | | | SCAC4 |
| 414 | UCX | United Coal Co. (Hillman Coal & Coke Co.) | 8/1920 | | NAKINA |
| 415 | UCX | United Coal Corp. (Cumberland & Pennsylvania R.R.) | 8/1920 | | NAKINA |
| 416 | UCX | Uvalde Construction Co. | 1/1941-4/1961 (not listed) | | NAKINA |
| 417 | UCX | Uvalde Construction Co. | 1/52-4/61 | | NRHS |
| 418 | UCX | CUMBERLAND & PENNSYLVANIA RAILROAD (UNITED COAL CORP.) [HILLMAN COAL & COKE CO.] | | | PWRC |
| 419 | UCXL | UNITED CROSSINGS TRANSPORTATION LLC | | | SCAC4 |
| 420 | UD | Hex'Air | ICAO: HER, Callsign: Hex Airline, Merged with Twin Jet during January 2017. | F | AVCODES |
| 421 | UD | Fast Air Ltda. | ICAO: FST, Callsign: Faster, Designator UD recalled by IATA | RCH | AVCODES |
| 422 | UD | Linea Aerea Sud-Americana ltda. (LASA) | Callsign: Lasa, | RCH | AVCODES |
| 423 | UD | Owenair (Pty) Ltd. | ICAO: OWE, Callsign: Owenair, Designator UD* recalled by IATA. | ZA | AVCODES |
| 424 | UD | UR Airlines d/b/a UBD | ICAO: UBD, Callsign: Urair, All flights operated by Aircompany Jonika LLC. (JNK) | IRQ | AVCODES |
| 425 | UDA | Pusdiklat Perhubungan Udara/Plp | Callsign: Udara, | RI | AVCODES |
| 426 | UDA | Psudiklat Perhubungan Udara/PLP | | | ICAO1 |
| 427 | UDA | Usafiri Dar es Salaam | | EAT | TRAFFIC |
| 428 | UDAX | United Distillers of America, Inc. | 1/1950 | | NAKINA |
| 429 | UDAX | North American Car Corp. | 1/1952 | | NAKINA |
| 430 | UDAX | North American Car Corp. | Jan-52 | | NRHS |
| 431 | UDAX | UNITED DISTILLERS OF AMERICA, INC. (NORTH AMERICAN CAR CORP.) | | | PWRC |
| 432 | UDC | Utility Enterprise DonbassAero Airline | IATA: 7D, Callsign: Aviation donbass, IATA Prefix: 897, Name changed from Aircompany Donbass - Eastern Ukrainian Airlines | UA | AVCODES |
| 433 | UDC | DonbassAero | IATA: 5D | | ICAO1 |
| 434 | UDC | UNION DEPOT COMPANY - COLUMBIA, OHIO | | | PWRC |
| 435 | UDCA | URIBE DURAN, CARLOS | | | SCAC4 |
| 436 | UDCP | UNION DISTRIBUTING COMPANY | | | SCAC4 |
| 437 | UDCS | UNITED ORCHIDS PLUS CORP | | | SCAC4 |
| 438 | UDCT | UNITED DRIVERS CORPORATION INC | | | SCAC4 |
| 439 | UDD | Air Company Donbass Airlines | IATA: 5D, Callsign: Donbass, IATA designator changed from 7D | UA | AVCODES |
| 440 | UDD | Air Company Donbass Airlines | IATA: 7D, Callsign: Donbass, IATA designator changed from 5D | UA | AVCODES |
| 441 | UDD | Air Company Donbass Airlines | IATA: 5D, Callsign: Donbass, Designator 5D recalled by IATA. IATA designator changed from 7D | UA | AVCODES |
| 442 | UDFC | UNITED FUEL & ENERGY CORPORATION | | | SCAC4 |
| 443 | UDFG | UNITED FOOD GROUP LLC | | | SCAC4 |
| 444 | UDFI | UNITED AIR LINES INC | | | SCAC4 |
| 445 | UDFT | UNITED DAIRY FARMERS TRANSIT INC | | | SCAC4 |
| 446 | UDG | Team 125, Inc. | Callsign: Inderdog, | USA | AVCODES |
| 447 | UDIET | Gleis- und Signalbau Uwe Dietrich | | DE | VKM |
| 448 | UDIP | UNITED DISPATCH INC | | | SCAC4 |
| 449 | UDIQ | URQUART DELIVERY INC | | | SCAC4 |
| 450 | UDIX | Utilities Distributors, Inc. | 1/1943 | | NAKINA |
| 451 | UDIX | Utilities Transport Co. | 1/1945 | | NAKINA |
| 452 | UDIX | Utilities Distributors Gas Co., Inc. | 1/1946-1/1950 | | NAKINA |
| 453 | UDIX | Utilities Distributors, Inc. | 1/1952; eliminated 7/1953 | | NAKINA |
| 454 | UDIX | Utilities Distributors, Inc. | Jan-52 | | NRHS |
| 455 | UDIX | UTILITIES DISTRIBUTORS, INC. [UTILITIES TRANSPORT CO.] | | | PWRC |
| 456 | UDLC | UPDIKE DISTRIBUTION LOGISTICS LLC | | | SCAC4 |
| 457 | UDMA | UNIVAR DE MEXICO SA DE CV | | | SCAC4 |
| 458 | UDN | Public JSC "Aviation Company " Dniproavia" | IATA: Z6, Callsign: Dniepro, IATA Prefix: 181, Declared bankrupt in Novmber 2017.Desigantor Z6* recalled by IATA. Name changed from Dniproavia - Joint Stock Aviation Co. | UA | AVCODES |
| 459 | UDN | Dniproavia | | | ICAO1 |
| 460 | UDNS | ULUSOY DENIZYOLLAAI ISLETMECILIGI A | | | SCAC4 |
| 461 | UDRM | UNIVERSAL DRUM RECONDITIONING CO | | | SCAC4 |
| 462 | UDRS | UNITED STATES DEPARTMENT OF AGRICUL | | | SCAC4 |
| 463 | UDRY | USA Logistics | | | SCAC2 |
| 464 | UDRY | USA DRY VAN LOGISTICS LLC | | | SCAC4 |
| 465 | UDSC | U.S. COURIER CORPORATION OF LOS ANGELES | | | SCAC1 |
| 466 | UDSC | U S COURIER CORPORATION OF LOS ANGE | | | SCAC4 |
| 467 | UDSE | U D S | | | SCAC4 |
| 468 | UDSL | UNITED DELIVERY SERVICE, LTD | | | SCAC1 |
| 469 | UDSL | UNITED DELIVERY SERVICE LTD | | | SCAC4 |
| 470 | UDSN | UDS INC | | | SCAC4 |
| 471 | UDSP | UNITED SPEED LOGISTICS LTD | | | SCAC4 |
| 472 | UDSS | UNIQUE DELIVERY SERVICE | | | SCAC4 |
| 473 | UDST | UNITED DISTRIBUTION SYSTEM INC | | | SCAC4 |
| 474 | UDSY | UNICITY DISTRIBUTION SYSTEMS (CANADA) INC | | | SCAC1 |
| 475 | UDTC | UP & DOWN TRUCKING | | | SCAC4 |
| 476 | UDTE | UNITED TRANSPORT SERVICES INC | | | SCAC4 |
| 477 | UDTI | U DIRECT TRANSPORTATION INC | | | SCAC4 |
| 478 | UDTK | UNCLE DONS TRUCKING | | | SCAC4 |
| 479 | UDTL | U DIRECT TRANSPORTATION | | | SCAC4 |
| 480 | UDTO | UNITED DRIVER TRANSPORT INC | | | SCAC4 |
| 481 | UDTP | UNION DE TRANSPORTISTAS DE VALLE HE | | | SCAC4 |
| 482 | UDTR | UD Trucks | Formerly Nissan Diesel Motor Co, now owned by Volvo. | J | TRAFFIC |
| 483 | UDTT | UNIDOS TRANSPORT | | | SCAC4 |
| 484 | UDTZ | UPPER DEERFIELD TOWNSHIP NEW JERSEY | | | PWRC |
| 485 | UDUI | UDUI TRUCKING LLC | | | SCAC4 |
| 486 | UDX | United Dairy Co. | 7/1935 | | NAKINA |
| 487 | UDX | United Dairy Co., Inc. | 7/1937-4/1938 | | NAKINA |
| 488 | UDX | United Dairy Co. | 1/1941 | | NAKINA |
| 489 | UDX | United Dairy Co. | Apr-38 | | NRHS |
| 490 | UDX | UNITED DAIRY CO., INC. | | | PWRC |
| 491 | UDYG | UNITED DRAYAGE | | | SCAC4 |
| 492 | UE | Transeuropean Airlines | ICAO: TEP, Callsign: Transeurline, | RUS | AVCODES |
| 493 | UE | Air Euroamerica S.A. (Uair) | | ROU | AVCODES |
| 494 | UE | Nasair Air Service | ICAO: NAS, Callsign: Nasairways, IATA Prefix: 770, | ER | AVCODES |
| 495 | UE | Ultimate Jetcharters, Inc. | ICAO: UJC, Callsign: Ultimate, Name changed to Ultimate Jetcharters LLC dba Ultimate Air Shuttle. IATA designator changed from UI. | USA | AVCODES |
| 496 | UE | Ultimate Jetcharters LLC dba Ultimate Air Shuttle | ICAO: UJC, Callsign: Ultimate, Name changed from Ultimate Jetcharters, Inc. IATA designator changed from UI. | USA | AVCODES |
| 497 | UE | Union Electric Ry. | 1/1946-7/1954 | | NAKINA |
| 498 | UE | Union Electric Ry. | Jan-52 | | NRHS |
| 499 | UE | UNION ELECTRIC RWY. | | | PWRC |
| 500 | UEA | United Eagle Airlines Co Ltd | IATA: EU, Callsign: United Eagle, IATA Prefix: 811, Name changed to Chengdu Airlines | PRC | AVCODES |
