| Siding Spring-LCO B | Australia | Coonabarabran | — |
| Siding Spring-LCO Clamshell #1 | Australia | Coonabarabran | — |
| Siding Spring-LCO Clamshell #2 | Australia | Coonabarabran | — |
| Sidoli observatory | Argentina | Luis Guillón | — |
| Siegen observatory | Alemania | Siegen | — |
| Siena observatory | España | Ager | — |
| Sierra Contraviesa observatory | España | Los Vargas | — |
| Sierra Remote observatory | Estados Unidos | Auberry | — |
| Sierra Sinagua Observatory | Estados Unidos | Flagstaff | — |
| Sierra Stars observatory | Estados Unidos | Mesa Vista | — |
| Signilskär Bird Observatory | Finlandia | Hammarland | — |
 Silesian Planetarium | Polonia | Chorzów | 316 m |
 Siligo astronomical observatory | Italia | Siligo | — |
| Silver Spring observatory | Estados Unidos | White Oak | — |
 SIM Lite Astrometric Observatory | — | — | — |
 Simplesat | — | — | — |
 Sinnott Memorial Observation Station | Estados Unidos | Klamath County | 2.027 m |
| Sinsen observatory | Alemania | Sinsen-Lenkerbeck | — |
| Sintini observatory | Italia | Alfonsine | — |
| Sirius observatory | España | Las Lomas | — |
 Sistema estereoscópico de alta energía | Namibia | Khomas Region | 1.800 m |
| Sitka Magnetic Observatory | Estados Unidos | — | — |
| SKA Telescope, Australia | Australia | Western Australia | — |
| Skibotn Astrophysical Observatory | Noruega | Skibotn | — |
 Skinakas Observatory | Grecia | — | 1.750 m |
| Skokholm Bird Observatory | Reino Unido | — | — |
| Sky Vistas observatory | Austria | Eichgraben | — |
 Sky's the Limit Observatory and Nature Center | Estados Unidos | Twentynine Palms | — |
| Skybor Observatory | España | Borja | — |
| Skygems Namibia Remote Observatory | Namibia | 82346 Hakos | — |
| Skyledge Observatory | Estados Unidos | Killingworth | — |
| Skylive observatory | Italia | Catania | — |
 SkyMapper | Australia | New South Wales | 1.163 m |
| Sliven Observatory | Bulgaria | Sliven | — |
| Slooh.com Canary Islands observatory | España | Parque Nacional del Teide | — |
| Slooh.com Chile Observatory | Chile | Lo Barnechea | — |
| Slope Rock observatory | Finlandia | Hyvinkää | — |
 Slottsskogsobservatoriet | Suecia | Municipalidad de Gothenburg | — |
 Slovak Central Observatory Hurbanovo | Eslovaquia | Hurbanovo | 115 m |
 Small Astronomy Satellite 2 | Estados Unidos | — | — |
 Small Astronomy Satellite 3 | Estados Unidos | — | — |
 Smeetoren | Países Bajos | Utrecht | — |
| Smith College Observatory | Estados Unidos | Northampton | — |
| Smith Observatory and Dr. William R. Brooks House | Estados Unidos | Nueva York | — |
| Smith River Observatory | Estados Unidos | Danbury | — |
| Smolecin observatory | Polonia | Smolęcin | — |
| Smolyan Planetarium | Bulgaria | Smolyan | — |
| Snohomish Hilltop Observatory | Estados Unidos | Snohomish | — |