| 1 | SI | Skynet Airlines Limited | ICAO: SIH, Callsign: Bluejet, IATA designator changed to SI | IRL | AVCODES |
| 2 | SI | Friesenflug GmbH & Co. KG | | D | AVCODES |
| 3 | SI | Sierra Pacific Airlines, Inc. | ICAO: SPA, Callsign: Sierra Pacific, Designator SI* reinstated by IATA | USA | AVCODES |
| 4 | SI | SaBer Air Private Limited | | SGP | AVCODES |
| 5 | SI | Skynet Airlines Limited | ICAO: SIH, Callsign: Bluejet, founded in 2001. Ceased operations in 2004. IATA designator changed from SI* | IRL | AVCODES |
| 6 | SI | Arab Wings Company | ICAO: AWS, Callsign: Arab Wings, Designator SI recalled by IATA | JOR | AVCODES |
| 7 | SI | Airx Limited t/a Blue Islands | ICAO: BCI, Callsign: Blue Island, Name changed from Airx Limited t/a Lecocqs.com and then from Airx Limited t/a Rockhopper. IATA designator changed from AX*. ICAO designator changed from XAX | GBJ | AVCODES |
| 8 | SI | Blue Islands Limited | ICAO: BCI, Callsign: Blue Island, Name changed from Airx Limited t/a Lecocqs.com and then from Airx Limited t/a Rockhopper. IATA designator changed from AX*. ICAO designator changed from XAX | GBJ | AVCODES |
| 9 | SI | Spokane International R.R. Co. | 1/1975-1/1980; added 1/1988 | | NAKINA |
| 10 | SI | Union Pacific R.R. Co. | added 4/1988; 1/1994; eliminated 4/1994; 7/2000-7/2005 | | NAKINA |
| 11 | SI | Spokane International R.R. Co. | 1/75-1/80; added 1/88 | | NRHS |
| 12 | SI | Union Pacific R.R. Co. | added 4/88; 1/94; eliminated 4/94; 7/2000 | | NRHS |
| 13 | SI | UNION PACIFIC RAILROAD CO. [SPOKANE INTERNATIONAL RR] | | | PWRC |
| 14 | SI | Spokane International Railroad (Union Pacific Railroad) | | | RSRV |
| 15 | SI | Spokane International R.R. Co. | | USA | TRAFFIC |
| 16 | SI | Sistemi Territoriali S.p.a. | | IT | VKM |
| 17 | SI&P | Southern Illinois Ry. and Power Co. | 7/1923-7/1930 | | NAKINA |
| 18 | SI&P | Southern Illinois Ry. & Power Co. | 7/1932 | | NAKINA |
| 19 | SI&P | SOUTHERN ILLINOIS RWY. & POWER CO. | | | PWRC |
| 20 | SIA | Singapore Airlines | IATA: SQ, Callsign: Singapore, IATA Prefix: 618, Name changed to Singapore Airlines Limited | SGP | AVCODES |
| 21 | SIA | Singapore Airlines Limited | IATA: SQ, Callsign: Singapore, IATA Prefix: 618, Name changed from Singapore Airlines | SGP | AVCODES |
| 22 | SIA | Singapore Airlines | IATA: SQ | | ICAO1 |
| 23 | SIA | Singapore Airlines Limited | | SGP | TRAFFIC |
| 24 | SIAA | SIERRAS ALCARAZ, ALEJANDRO | | | SCAC4 |
| 25 | SIAD | SANTSAR INDUSTRIES INC | | | SCAC4 |
| 26 | SIAG | SATILLA TRANSPORT INC | | | SCAC4 |
| 27 | SIAL | SIMMONS, L TRUCKING LLC | | | SCAC4 |
| 28 | SIAM | SCHNEIDERS IRON & METAL INC | | | SCAC4 |
| 29 | SIAN | SIARC INC | | | SCAC4 |
| 30 | SIAP | SILVERTRANS INC | | | SCAC4 |
| 31 | SIAR | SIMBA LOGISTICS LTD | | | SCAC4 |
| 32 | SIAT | SST INTERNATIONAL INC | | | SCAC4 |
| 33 | SIAU | SIAD SOCIETA ITALIANA ACETILENE & DERIVATI SPA | | I | BIC |
| 34 | SIAU | SIAD SOCIETA ITALIANA ACETILENE & D | | | SCAC4 |
| 35 | SIAV | SIMPSON ENTERPRISES LTD | | | SCAC4 |
| 36 | SIAW | SILVER ARROW, INC | | | SCAC1 |
| 37 | SIAW | SILVER ARROW INC | | | SCAC4 |
| 38 | SIAY | SOLIDARITE 2000 INC | | | SCAC4 |
| 39 | SIB | Joint Stock Company SIAT (Sibaviatrans) | IATA: 5M, Callsign: Sibavia, IATA Prefix: 842, Suspended operations at the end of September 2008 due to the Air Union collapse. | RUS | AVCODES |
| 40 | SIB | Siba S.p.A. | | I | AVCODES |
| 41 | SIB | Sea Air Ltd. | Callsign: Sea Air, | SLO | AVCODES |
| 42 | SIB | Sibaviatrans | IATA: 5M | | ICAO1 |
| 43 | SIBB | SOUTHWESTERN ILLINOIS BUS COMPANY L | | | SCAC4 |
| 44 | SIBD | SIMON TRANSPORT LLC (BROKERAGE DIVI | | | SCAC4 |
| 45 | SIBG | SIT | | | SCAC4 |
| 46 | SIBI | SISBRO, INC | | | SCAC1 |
| 47 | SIBI | SISBRO INC | | | SCAC4 |
| 48 | SIBIU | Sibiu: Tursib | | RO | TRAFFIC |
| 49 | SIBN | SIBRIAN TRUCKING | | | SCAC4 |
| 50 | SIBP | SIMBA TRANSPORT LTD | | | SCAC4 |
| 51 | SIBQ | SERVICES INTERNATIONAL | | | SCAC4 |
| 52 | SIBRA | Société Intercommunale des Bus de la Région d'Annecy | City: Annecy | F | TRAFFIC |
| 53 | SIBS | SIMPSON FISHING CO LTD | | | SCAC4 |
| 54 | SIBT | SIBLEY TRUCKING INC | | | SCAC4 |
| 55 | SIBU | GE SIMONS INTERNATIONAL TRANSPORT BV | | NL | BIC |
| 56 | SIBU | GE SIMONS INTERNATIONAL TRANSPORT B | | | SCAC4 |
| 57 | SIBV | SCHILLI DISTRIBUTION SERVICES INC | | | SCAC4 |
| 58 | SIBZD | Siberian Railway | | RUS | TRAFFIC |
| 59 | SIC | Air Sicilia S.p.A. | IATA: BM, Callsign: Air Sicilia, Ceased operations during 2004. Name changed from Air Sicilia S.R.L. | I | AVCODES |
| 60 | SIC | Air Sicilia S.R.L. | IATA: BM, Callsign: Air Sicilia, Name changed to Air Sicilia S.p.A. | I | AVCODES |
| 61 | SICC | SIX ZERO THREE SIX SIX FIVE ONE (60 | | | SCAC4 |
| 62 | SICD | SPECTRAN, INC | | | SCAC1 |
| 63 | SICF | SLOAN IMPLEMENT COMPANY | | | SCAC4 |
| 64 | SICG | SICKLER'S TRUCKING | | | SCAC4 |
| 65 | SICH | STORM INTERSTATE CO | | | SCAC4 |
| 66 | SICJ | SILVER CITY ALUMINUM CORP | | | SCAC4 |
| 67 | SICL | SILVER CITY LUMBER INC | | | SCAC4 |
| 68 | SICO | SIMPSON EXPRESS AND TRANSFER INC | | | SCAC4 |
| 69 | SICR | SCHAKE INDUSTRIES INC | | | SCAC4 |
| 70 | SICT | SV TRUCKING SERVICES LLC | | | SCAC4 |
| 71 | SICU | SHANGHAI XURONG CONTAINERCO., LTD | | PRC | BIC |
| 72 | SICU | SIC PROCESSING GMBH | | | SCAC4 |
| 73 | SICW | SICOMSA | | | SCAC4 |
| 74 | SICX | Sloss Industries Corp. | added 7/1988; 1/1994-7/2000; eliminated 1/2005; 7/2005 (not listed) | | NAKINA |
| 75 | SICX | Sloss Industries Corp. | added 7/88; 1/94-7/2000 | | NRHS |
| 76 | SICX | SLOSS INDUSTRIES CORPORATION | | | PWRC |
| 77 | SICX | Sloss Industries Corporation | | | RSRV |
| 78 | SICX | SLOSS INDUSTRIES CORPORATION | | | SCAC4 |
| 79 | SID | Sidfin Air Limited | Callsign: Sidfin, | Z | AVCODES |
| 80 | SID | Sideral Linhas Aereas Ltda | Callsign: Sideral, | BR | AVCODES |
| 81 | SIDB | SIDHU BROTHERS INC | | | SCAC4 |
| 82 | SIDC | SIMONSEN TRUCKING | | | SCAC4 |
| 83 | SIDE | SCHEIDERER TRANSPORT INC | | | SCAC4 |
| 84 | SIDER | Siderpotenza Steel Works | | I | TRAFFIC |
| 85 | SIDG | SIEDHOFF DISTRIBUTING CO | | | SCAC4 |
| 86 | SIDH | SIDHU SERVICES LTD | | | SCAC4 |
| 87 | SIDI | SHANE TRANSPORT, Shane Industries, INC | | | SCAC1 |
| 88 | SIDI | SHANE TRANSPORT | | | SCAC4 |
| 89 | SIDM | SINDALL TRANSPORT INC | | | SCAC4 |
| 90 | SIDO | SERVICIOS INTEGRALES DE TRANSPORTES | | | SCAC4 |
| 91 | SIDQ | STAR INDUSTRIES LLC | | | SCAC4 |
| 92 | SIDS | SUPERIOR INTERMODAL SERVICES INC | | | SCAC4 |
| 93 | SIDU | CHEMIKALIEN UND FLUESSIGKEITSTRANSPORTE A. SIEPMANN GMBH | | D | BIC |
| 94 | SIDY | SINANO SHIPMANAGEMENT LIMITED | | | SCAC4 |
| 95 | SIE | Sierra Express | Callsign: Serex, | USA | AVCODES |
| 96 | SIE | Sierra Express | | | ICAO1 |
| 97 | SIE | Siemens | | D | TRAFFIC |
| 98 | SIEA | SIERRA AGRICULTURAL TRANSPORTATION | | | SCAC4 |
| 99 | SIEB | SCHIELE ENTERPRISES INC | | | SCAC4 |
| 100 | SIEB | | City: Siegburg | D | TRAFFIC |
