| 1 | UD | Hex'Air | ICAO: HER, Callsign: Hex Airline, Merged with Twin Jet during January 2017. | F | AVCODES |
| 2 | UD | Fast Air Ltda. | ICAO: FST, Callsign: Faster, Designator UD recalled by IATA | RCH | AVCODES |
| 3 | UD | Linea Aerea Sud-Americana ltda. (LASA) | Callsign: Lasa, | RCH | AVCODES |
| 4 | UD | Owenair (Pty) Ltd. | ICAO: OWE, Callsign: Owenair, Designator UD* recalled by IATA. | ZA | AVCODES |
| 5 | UD | UR Airlines d/b/a UBD | ICAO: UBD, Callsign: Urair, All flights operated by Aircompany Jonika LLC. (JNK) | IRQ | AVCODES |
| 6 | UDA | Pusdiklat Perhubungan Udara/Plp | Callsign: Udara, | RI | AVCODES |
| 7 | UDA | Psudiklat Perhubungan Udara/PLP | | | ICAO1 |
| 8 | UDA | Usafiri Dar es Salaam | | EAT | TRAFFIC |
| 9 | UDAX | United Distillers of America, Inc. | 1/1950 | | NAKINA |
| 10 | UDAX | North American Car Corp. | 1/1952 | | NAKINA |
| 11 | UDAX | North American Car Corp. | Jan-52 | | NRHS |
| 12 | UDAX | UNITED DISTILLERS OF AMERICA, INC. (NORTH AMERICAN CAR CORP.) | | | PWRC |
| 13 | UDC | Utility Enterprise DonbassAero Airline | IATA: 7D, Callsign: Aviation donbass, IATA Prefix: 897, Name changed from Aircompany Donbass - Eastern Ukrainian Airlines | UA | AVCODES |
| 14 | UDC | DonbassAero | IATA: 5D | | ICAO1 |
| 15 | UDC | UNION DEPOT COMPANY - COLUMBIA, OHIO | | | PWRC |
| 16 | UDCA | URIBE DURAN, CARLOS | | | SCAC4 |
| 17 | UDCP | UNION DISTRIBUTING COMPANY | | | SCAC4 |
| 18 | UDCS | UNITED ORCHIDS PLUS CORP | | | SCAC4 |
| 19 | UDCT | UNITED DRIVERS CORPORATION INC | | | SCAC4 |
| 20 | UDD | Air Company Donbass Airlines | IATA: 5D, Callsign: Donbass, IATA designator changed from 7D | UA | AVCODES |
| 21 | UDD | Air Company Donbass Airlines | IATA: 7D, Callsign: Donbass, IATA designator changed from 5D | UA | AVCODES |
| 22 | UDD | Air Company Donbass Airlines | IATA: 5D, Callsign: Donbass, Designator 5D recalled by IATA. IATA designator changed from 7D | UA | AVCODES |
| 23 | UDFC | UNITED FUEL & ENERGY CORPORATION | | | SCAC4 |
| 24 | UDFG | UNITED FOOD GROUP LLC | | | SCAC4 |
| 25 | UDFI | UNITED AIR LINES INC | | | SCAC4 |
| 26 | UDFT | UNITED DAIRY FARMERS TRANSIT INC | | | SCAC4 |
| 27 | UDG | Team 125, Inc. | Callsign: Inderdog, | USA | AVCODES |
| 28 | UDIET | Gleis- und Signalbau Uwe Dietrich | | DE | VKM |
| 29 | UDIP | UNITED DISPATCH INC | | | SCAC4 |
| 30 | UDIQ | URQUART DELIVERY INC | | | SCAC4 |
| 31 | UDIX | Utilities Distributors, Inc. | 1/1943 | | NAKINA |
| 32 | UDIX | Utilities Transport Co. | 1/1945 | | NAKINA |
| 33 | UDIX | Utilities Distributors Gas Co., Inc. | 1/1946-1/1950 | | NAKINA |
| 34 | UDIX | Utilities Distributors, Inc. | 1/1952; eliminated 7/1953 | | NAKINA |
| 35 | UDIX | Utilities Distributors, Inc. | Jan-52 | | NRHS |
| 36 | UDIX | UTILITIES DISTRIBUTORS, INC. [UTILITIES TRANSPORT CO.] | | | PWRC |
| 37 | UDLC | UPDIKE DISTRIBUTION LOGISTICS LLC | | | SCAC4 |
| 38 | UDMA | UNIVAR DE MEXICO SA DE CV | | | SCAC4 |
| 39 | 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 |
| 40 | UDN | Dniproavia | | | ICAO1 |
| 41 | UDNS | ULUSOY DENIZYOLLAAI ISLETMECILIGI A | | | SCAC4 |
| 42 | UDRM | UNIVERSAL DRUM RECONDITIONING CO | | | SCAC4 |
| 43 | UDRS | UNITED STATES DEPARTMENT OF AGRICUL | | | SCAC4 |
| 44 | UDRY | USA Logistics | | | SCAC2 |
| 45 | UDRY | USA DRY VAN LOGISTICS LLC | | | SCAC4 |
| 46 | UDSC | U.S. COURIER CORPORATION OF LOS ANGELES | | | SCAC1 |
| 47 | UDSC | U S COURIER CORPORATION OF LOS ANGE | | | SCAC4 |
| 48 | UDSE | U D S | | | SCAC4 |
| 49 | UDSL | UNITED DELIVERY SERVICE, LTD | | | SCAC1 |
| 50 | UDSL | UNITED DELIVERY SERVICE LTD | | | SCAC4 |
| 51 | UDSN | UDS INC | | | SCAC4 |
| 52 | UDSP | UNITED SPEED LOGISTICS LTD | | | SCAC4 |
| 53 | UDSS | UNIQUE DELIVERY SERVICE | | | SCAC4 |
| 54 | UDST | UNITED DISTRIBUTION SYSTEM INC | | | SCAC4 |
| 55 | UDSY | UNICITY DISTRIBUTION SYSTEMS (CANADA) INC | | | SCAC1 |
| 56 | UDTC | UP & DOWN TRUCKING | | | SCAC4 |
| 57 | UDTE | UNITED TRANSPORT SERVICES INC | | | SCAC4 |
| 58 | UDTI | U DIRECT TRANSPORTATION INC | | | SCAC4 |
| 59 | UDTK | UNCLE DONS TRUCKING | | | SCAC4 |
| 60 | UDTL | U DIRECT TRANSPORTATION | | | SCAC4 |
| 61 | UDTO | UNITED DRIVER TRANSPORT INC | | | SCAC4 |
| 62 | UDTP | UNION DE TRANSPORTISTAS DE VALLE HE | | | SCAC4 |
| 63 | UDTR | UD Trucks | Formerly Nissan Diesel Motor Co, now owned by Volvo. | J | TRAFFIC |
| 64 | UDTT | UNIDOS TRANSPORT | | | SCAC4 |
| 65 | UDTZ | UPPER DEERFIELD TOWNSHIP NEW JERSEY | | | PWRC |
| 66 | UDUI | UDUI TRUCKING LLC | | | SCAC4 |
| 67 | UDX | United Dairy Co. | 7/1935 | | NAKINA |
| 68 | UDX | United Dairy Co., Inc. | 7/1937-4/1938 | | NAKINA |
| 69 | UDX | United Dairy Co. | 1/1941 | | NAKINA |
| 70 | UDX | United Dairy Co. | Apr-38 | | NRHS |
| 71 | UDX | UNITED DAIRY CO., INC. | | | PWRC |
| 72 | UDYG | UNITED DRAYAGE | | | SCAC4 |
