| 1 | UN | Transaero Airlines | ICAO: TSO, Callsign: Transoviet, IATA Prefix: 670, Founded on 28 December 1990. Commenced operations on 05 November 1991. Ceased operations on 25 October 2015. | RUS | AVCODES |
| 2 | UN | Universair, SA | ICAO: UNA, Callsign: Universair, Commenced operations in 1986. Merged into Meridiana Air, SA on 01 March 1991. | E | AVCODES |
| 3 | UN | East Coast Airlines Limited | | AUS | AVCODES |
| 4 | UN | Business Aviation Asia Limited | | HK | AVCODES |
| 5 | UN | Business Aviation Asia Limited | ICAO: UNA, | HK | AVCODES |
| 6 | UNA | Universair, SA | IATA: UN, Callsign: Universair, Commenced operations in 1986. Merged into Meridiana Air, SA on 01 March 1991. | E | AVCODES |
| 7 | UNA | Business Aviation Asia Limited | IATA: UN, | HK | AVCODES |
| 8 | UNAA | UNIQUE FREIGHT | | | SCAC4 |
| 9 | UNAC | UNITED FUEL SERVICE | | | SCAC4 |
| 10 | UNACOP | Unacop d.o.o. | City: Beograd | SR | TRAFFIC |
| 11 | UNAP | UNITED EXPRESS LTD | | | SCAC4 |
| 12 | UNAR | UNION AIR TRANSPORT GMBH | | | SCAC4 |
| 13 | UNAS | UNION AGRICOLA S A DE C V | | | SCAC4 |
| 14 | UNAU | UNITEAM AUSTRALASIA PTY LTD | | | SCAC4 |
| 15 | UNBC | UNITED BULB COMPANY | | | SCAC4 |
| 16 | UNBD | UNIONVILLEDEPOT.COM | | | SCAC4 |
| 17 | UNBL | UNITED BLOWER INC | | | SCAC4 |
| 18 | UNBX | Union Boiler Repair, Fabrication & Erection Co., Inc. | added 7/1970; eliminated 10/1972 | | NAKINA |
| 19 | UNBX | Union Boiler Repair | added 10/1972; eliminated 11/1973 | | NAKINA |
| 20 | UNBX | General Electric Railcar Services Corp. | added 4/1999; eliminated 1/2000 | | NAKINA |
| 21 | UNBX | General Electric Rail Services Corp. | added 1/2000; 7/2000-7/2005 | | NAKINA |
| 22 | UNBX | Union Boiler Repair, Fabrication & Erection Co., Inc. | added 7/70; eliminated 10/72 | | NRHS |
| 23 | UNBX | Union Boiler Repair | added 10/72; eliminated 11/73 | | NRHS |
| 24 | UNBX | General Electric Railcar Services Corp. | added 4/99; eliminated 1/2000 | | NRHS |
| 25 | UNBX | General Electric Rail Services Corp. | added 1/2000; 7/2000 | | NRHS |
| 26 | UNBX | *UNION BOILER REPAIR | | | PWRC |
| 27 | UNBX | +GENERAL ELECTRIC RAIL SERVICES CORP. | | | PWRC |
| 28 | UNBX | General Electric Rail Services Corporation | | | RSRV |
| 29 | UNBX | GENERAL ELECTRIC RAILCAR SERVICES C | | | SCAC4 |
| 30 | UNC | Uni-fly A/S | Callsign: Unicopter, | DK | AVCODES |
| 31 | UNC | Uni-Fly | | | ICAO1 |
| 32 | UNCA | UNKNOWN CANADIAN AIR CARRIER | | | SCAC1 |
| 33 | UNCA | UNITED CAB ASSOCIATION | | | SCAC4 |
| 34 | UNCB | UNITED TOTE COMPANY | | | SCAC4 |
| 35 | UNCC | UNICOLD CORPORATION | | | SCAC4 |
| 36 | UNCD | UNICEL LIMITED | | | SCAC4 |
| 37 | UNCF | UNISEA SHIPPING CV | | | SCAC4 |
| 38 | UNCG | UNION CARTAGE CO | | | SCAC1 |
| 39 | UNCH | UNICHAIRS INC | | | SCAC4 |
| 40 | UNCL | UNITED CONTAINERLINES INC | | | SCAC4 |
| 41 | UNCR | UNITED CRANE & RIGGING | | | SCAC4 |
| 42 | UNCU | UNI-LOGISTICS SP. Z O.O. | | PL | BIC |
| 43 | UNCU | UNIVERSAL CONTAINER LTD | | | SCAC4 |
| 44 | UNCX | United Central Oil Corp. (Crown Central Petroleum Corp.) | 7/1930 | | NAKINA |
| 45 | UNCX | Universal Car Co. | 7/1935-4/1938 | | NAKINA |
| 46 | UNCX | Union Carbide Canada Ltd., Plastics & Chemicals | added 7/1981; eliminated 4/1987 | | NAKINA |
| 47 | UNCX | General Electric Railcar Services Corp. | eliminated 4/1992 | | NAKINA |
| 48 | UNCX | CGTX, Inc. | added 4/1992; 1/1994-7/2000; eliminated 4/2002 | | NAKINA |
| 49 | UNCX | GATX Rail Canada Corp. | added 4/2002; 7/2005 | | NAKINA |
| 50 | UNCX | Universal Car Co. | Apr-38 | | NRHS |
| 51 | UNCX | Union Carbide Canada Ltd., Plastics & Chemicals | added 7/81; eliminated 4/87 | | NRHS |
| 52 | UNCX | General Electric Railcar Services Corp. | eliminated 4/92 | | NRHS |
| 53 | UNCX | CGTX, Inc. | added 4/92; 1/94-7/2000 | | NRHS |
| 54 | UNCX | *CROWN CENTRAL PETROLEUM CO. [UNITED CENTRAL OIL CORP.] | | | PWRC |
| 55 | UNCX | *UNIVERSAL CAR CO. | | | PWRC |
| 56 | UNCX | *UNION CARBIDE CANADE, LTD. | | | PWRC |
| 57 | UNCX | +GATX RAIL CANADA CORP. [(CGTX)GENERAL ELECTRIC RAILCAR SERVICES CORP.] | | | PWRC |
| 58 | UNCX | Union Carbide Canada | | | RSRV |
| 59 | UNCX | GATX RAIL CANADA CORPORATION | | | SCAC4 |
| 60 | UNCZ | UNIVERSAL COOPERATIVE | | | PWRC |
| 61 | UND | Atuneros Unidos de California, S.A. de C.V. | Callsign: Atuneros Unidos, | MEX | AVCODES |
| 62 | UND | Atuneros Unidos de California | | | ICAO1 |
| 63 | UNDB | UNITED OCEAN LINES | | | SCAC4 |
| 64 | UNDG | UNITED LOGISTIC INC | | | SCAC4 |
| 65 | UNDH | UNDERWOOD, JEFF INC | | | SCAC4 |
| 66 | UNDI | UNITED OIL | | | SCAC4 |
| 67 | UNDO | UNITED OIL PACKERS INC | | | SCAC4 |
| 68 | UNDU | UNITAINER TRADING GMBH | | D | BIC |
| 69 | UNDU | UNITAINER TRADING GMBH | | | SCAC4 |
| 70 | UNDV | UNRAU, ABE J | | | SCAC4 |
| 71 | UNDY | UNIQUE TRUCKING LTD | | | SCAC4 |
| 72 | UNE | Aerolineas Universal S.A. | Callsign: Aeroune, Never started operations in 2001. | CO | AVCODES |
| 73 | UNEC | UNIVERSITY CORP | | | SCAC1 |
| 74 | UNER | UNITED EXPRESS | | | SCAC4 |
| 75 | UNES | UNITY EXPRESS | | | SCAC4 |
| 76 | UNEX | UNEX a.s. | | CZ | VKM |
| 77 | UNEZ | UNION ELECTRIC | | | PWRC |
| 78 | UNF | Union Flights | Callsign: Union Flights, | USA | AVCODES |
| 79 | UNF | Union Flights | | | ICAO1 |
| 80 | UNFA | UNKNOWN FOREIGN AIR CARRIER | | | SCAC1 |
| 81 | UNFC | UNIVERSAL FOODS CORPORATION | | | SCAC1 |
| 82 | UNFC | UNION AFRICA LINE | | | SCAC4 |
| 83 | UNFE | UNI-FAB ENCLOSURES | | | SCAC4 |
| 84 | UNFF | UNITED TRAFFIC SERVICES INC | | | SCAC4 |
| 85 | UNFN | UNITED INTERMODE INC | | | SCAC4 |
| 86 | UNFO | UNITED FOODS INC (FREEZER QUEEN FOO | | | SCAC4 |
| 87 | UNFQ | UNFI | | | SCAC4 |
| 88 | UNFU | UNIFEEDER FZCO | | UAE | BIC |
| 89 | UNFW | UNION FREIGHTWAYS | | | SCAC4 |
| 90 | UNFX | American Railcar Industries | added 10/2006 | | NAKINA |
| 91 | UNFX | AMERICAN RAILCAR INDUSTRIES | | | PWRC |
| 92 | UNFX | AMERICAN RAILCAR INDUSTRIES | | | SCAC4 |
| 93 | UNGD | UNITED GLOBAL LOGISTICS INC | | | SCAC4 |
| 94 | UNGI | UNIGAS INTERNATIONAL | | | SCAC4 |
| 95 | UNGL | UNIGLORY | | | SCAC1 |
| 96 | UNGN | UNIQUE TOOL & GAUGE INC | | | SCAC4 |
| 97 | UNHS | UNISHIPPERS | | | SCAC4 |
| 98 | UNI | United Air Charters | Callsign: Unitair, ICAO designator changed to UAC | ZW | AVCODES |
| 99 | UNI | Community Express Airlines Ltd. | IATA: 5V, Callsign: Community, Ceased operations. | GB | AVCODES |
| 100 | UNI | Universal Airlines | LCC of Air Europa | E | AVCODES |
