| 1 | SL | Rio Sul Linhas Aereos, S.A. | ICAO: RSL, Callsign: Riossul, IATA Prefix: 293, Merged with Nordeste and then into Varig. Name changed from Rio-Sul Servicos Aereos Regionais, S.A. Designator SL reinstated by IATA | BR | AVCODES |
| 2 | SL | Rio-Sul Servicos Aereos Regionais, S.A. | ICAO: RSL, Callsign: Riossul, IATA Prefix: 293, Name changed to Rio Sul Linhas Aereas S.A. Designator SL reinstated by IATA | BR | AVCODES |
| 3 | SL | Southeast Airlines, Inc. | ICAO: SNK, Callsign: Southeast, Commenced operations in 1999. Ceased operations on 30 November 2004 | USA | AVCODES |
| 4 | SL | Caribbean Skyways St. Lucia Express | ICAO: CSC, Callsign: St. Lucia Express, | WL | AVCODES |
| 5 | SL | Air Sylhet AG | ICAO: SHR, Founded in 2007. Commenced operations on 04 April 2009. Ceased operations operations in May 2009. | A | AVCODES |
| 6 | SL | Air Atlantic Sarl | ICAO: AIA, | P | AVCODES |
| 7 | SL | Thai Lion Mentari Co. Ltd. | ICAO: TLM, Callsign: Mentari, | T | AVCODES |
| 8 | SL | Scandinavian Airlines Ireland Limited | ICAO: SZS, Callsign: Spinnaker, Name changed to Scandinavian Airlines Ireland Limited d/b/a SAS Connect w.e.f. 28 September 2021. | IRL | AVCODES |
| 9 | SL | Scandinavian Airlines Ireland Limited d/b/a SAS Connect | ICAO: SZS, Callsign: Spinnaker, Name changed from Scandinavian Airlines Ireland Limited w.e.f. 28 September 2021. | IRL | AVCODES |
| 10 | SL | Sugar Land Ry. | 4/1938-7/1954 | | NAKINA |
| 11 | SL | Salt Lake City Southern R.R. Co., Inc. | 1/1994-7/2005 | | NAKINA |
| 12 | SL | Sugar Land Ry. | 4/38-1/52 | | NRHS |
| 13 | SL | Salt Lake City Southern R.R. Co., Inc. | 1/94-7/2000 | | NRHS |
| 14 | SL | *SUGAR LAND RWY. | | | PWRC |
| 15 | SL | +SALT LAKE CITY SOUTHERN RAILROAD COMPANY, INC. | | | PWRC |
| 16 | SL | Salt Lake City Southern Railroad | | | RSRV |
| 17 | SL | SALT LAKE CITY SOUTHERN RAILROAD CO | | | SCAC4 |
| 18 | SL | Storstockholms Lokaltrafiks | Real: SS, City: Stockholm | S | TRAFFIC |
| 19 | SL | Storstockhoms Lokaltrafik | | SE | VKM |
| 20 | SL&E | Sligo & Eastern R.R. | 7/1923 | | NAKINA |
| 21 | SL&E | SLIGO & EASTERN RAILROAD | | | PWRC |
| 22 | SL&H | St. Louis & Hannibal R.R. Co. | 7/1923-4/1938 | | NAKINA |
| 23 | SL&H | St. Louis & Hannibal R.R. Co. | Apr-38 | | NRHS |
| 24 | SL&H | ST. LOUIS & HANNIBAL RAILROAD CO. | | | PWRC |
| 25 | SL&L | Salt Lake, Garfield & Western Ry. Co. | 4/1938 | | NAKINA |
| 26 | SL&L | Salt Lake, Garfield & Western Ry. Co. | Apr-38 | | NRHS |
| 27 | SL&L | SALT LAKE, GARFIELD & WESTERN RWY. CO. | | | PWRC |
| 28 | SL&U | Salt Lake & Utah R.R. Co. | 7/1923-7/1936 | | NAKINA |
| 29 | SL&U | SATL LAKE & UTAH RAILROAD CO. | | | PWRC |
| 30 | SLA | Sierra National Airlines | IATA: LJ, Callsign: Selair, IATA Prefix: 690, Designator LJ recalled by IATA | WAL | AVCODES |
| 31 | SLA | Sierra National Airlines | IATA: LJ | | ICAO1 |
| 32 | SLAB | SCIERIE LABRANCHE INC | | | SCAC4 |
| 33 | SLAC | ST LAWRENCE AND ATLANTIC (QUEBEC) R | | | SCAC4 |
| 34 | SLAF | Sri Lanka Air Force | | CL | TRAFFIC |
| 35 | SLAG | STATELINE AG SERVICE INC | | | SCAC4 |
| 36 | SLAL | South Plains Lamesa R.R., Ltd. | 7/2000-7/2005 | | NAKINA |
| 37 | SLAL | South Plains Lamesa R.R., Ltd. | Jul-00 | | NRHS |
| 38 | SLAL | SOUTH PLAINS LAMESA RAILROAD, LTD. | | | PWRC |
| 39 | SLAL | South Plains Lamesa Railroad | | | RSRV |
| 40 | SLAL | SOUTH PLAINS LAMESA RAILROAD LTD | | | SCAC4 |
| 41 | SLAN | | City: Slanska Ruda | PL | TRAFFIC |
| 42 | SLANC | South Lancs Travel | | GB | TRAFFIC |
| 43 | SLAO | S.L.A. TRANSPORT, INC | | | SCAC1 |
| 44 | SLAQ | SALT EXCHANGE | | | SCAC4 |
| 45 | SLAR | STELLAR FREIGHT LIMITED | | | SCAC1 |
| 46 | SLAR | STELLAR LINES S A | | | SCAC4 |
| 47 | SLAS | SUNSPROUTS OF LANCASTER COUNTY | | | SCAC4 |
| 48 | SLAT | SALLEE EXPRESS | | | SCAC4 |
| 49 | SLAW | St. Lawrence R.R., Division of National Railway Utilization Corp. | 1/1980 | | NAKINA |
| 50 | SLAW | St. Lawrence R.R. | eliminated 7/1990 | | NAKINA |
| 51 | SLAW | St. Lawrence R.R., Division of National Railway Utilization Corp. | Jan-80 | | NRHS |
| 52 | SLAW | St. Lawrence R.R. | eliminated 7/90 | | NRHS |
| 53 | SLAW | ST. LAWRENCE RAILROAD, DIV. NATIONAL RAILWAY UTILIZATION CORP. | | | PWRC |
| 54 | SLAW | St. Lawrence Railroad | | | RSRV |
| 55 | SLAW | SECURITY LOGISTICS LLC | | | SCAC4 |
| 56 | SLAX | St. Lawrence Starch Co. Ltd. | 7/1937-1/1980; eliminated 10/1987 | | NAKINA |
| 57 | SLAX | St. Lawrence Starch Co. Ltd. | 4/38-1/80; eliminated 10/87 | | NRHS |
| 58 | SLAX | ST. LAWRENCE STARCH CO., LTD. | | | PWRC |
| 59 | SLAX | St. Lawrence Starch | | | RSRV |
| 60 | SLAY | SLAY TRANSPORTATION CO | | | SCAC1 |
| 61 | SLAY | SLAY TRANSPORTATION CO INC | | | SCAC4 |
| 62 | SLAZ | SLA PROPERTY MANAGEMENT LIMITED PARTNERSHIP | | | PWRC |
| 63 | SLB | Slok Air Limited | Callsign: Slok Air, | WAN | AVCODES |
| 64 | SLB | Slok Air | | | ICAO1 |
| 65 | SLB | Salzburger Lokalbahn | | A | TRAFFIC |
| 66 | SLB | Salzburg AG für Energie, Verkehr und Telekommunikation - Salzburger Lokalbahn | | AT | VKM |
| 67 | SLBB | SHOWA LINE, LTD. | | | PWRC |
| 68 | SLBF | STORAGE SOLUTIONS | | | SCAC4 |
| 69 | SLBI | SCL BASILEA AG | | | SCAC4 |
| 70 | SLBK | SHOWA LINE, LTD. | | | PWRC |
| 71 | SLBL | SLOTLABEL INC | | | SCAC4 |
| 72 | SLBN | STRATTON LUMBER INC | | | SCAC4 |
| 73 | SLBO | SCHUTTE LUMBER COMPANY | | | SCAC4 |
| 74 | SLBQ | SELDEN TRUCKING INC | | | SCAC4 |
| 75 | SLBT | SILVER BULLET TRUCKING | | | SCAC4 |
| 76 | SLBU | SHOWA LINE, LTD. | | | PWRC |
| 77 | SLBU | Showa Line Limited | | | RSRV |
| 78 | SLBV | STERLING HELICOPTER | | | SCAC4 |
| 79 | SLBY | S L B TRANSIT INC | | | SCAC4 |
| 80 | SLC | Swiftlines Limited | Callsign: Swiftlines, | EAK | AVCODES |
| 81 | SLC | Salsa D'Haiti | IATA: SO, Ceased operations | RH | AVCODES |
| 82 | SLC | San Luis Central R.R. | 1/1941-7/1954 | | NAKINA |
| 83 | SLC | The San Luis Central R.R. Co. | 1/1975-7/2005 | | NAKINA |
| 84 | SLC | San Luis Central R.R. | Jan-52 | | NRHS |
| 85 | SLC | The San Luis Central R.R. Co. | 1/75-7/2000 | | NRHS |
| 86 | SLC | SAN LUIS CENTRAL RAILROAD CO. | | | PWRC |
| 87 | SLC | San Luis Central Railroad | | | RSRV |
| 88 | SLC | San Luis Central R.R. | | USA | TRAFFIC |
| 89 | SLCA | S & L CARRIERS INC | | | SCAC4 |
| 90 | SLCB | SELECTIVE DISTRIBUTION LLC | | | SCAC4 |
| 91 | SLCD | SCHNEIDER LOGISTICS INC (INTEGRATED | | | SCAC4 |
| 92 | SLCF | SHOWA LINE, LTD. | | | PWRC |
| 93 | SLCF | SPEED LIGHT COURIER & LTL CO INC | | | SCAC4 |
| 94 | SLCF | SALCEF S.p.A. | | IT | VKM |
| 95 | SLCP | SEA LAUNCH COMPANY LLC | | | SCAC4 |
| 96 | SLCR | ST. LOUIS & CHAIN OF ROCKS RAILROAD | | | PWRC |
| 97 | SLCT | SEALAND CONSTRUCTION LTEE | | | SCAC4 |
| 98 | SLCU | SPECTRA TANKTAINER LOGISTICS PTE. LTD | | SGP | BIC |
| 99 | SLCU | *SALT LAKE CITY UNION DEPOT & RAILWAY CO. | | | PWRC |
| 100 | SLCU | *SLC SOCIETE LOCATION CONTAINERS | | | PWRC |
