| San Pablo-Toledo Magnetic Observatory | España | — | — |
| San Pedro Valley Observatory | Estados Unidos | Benson | — |
| San Pere observatory | España | Mataró | — |
| San Polo A Mosciano observatory | Italia | Scandicci | — |
| Sanderphil Urban Observatory | Italia | Port of Civitavecchia | — |
| Sandia View observatory | Estados Unidos | Río Rancho | — |
| Sandlot observatory | Estados Unidos | Kansas | — |
| Sandvreten Observatory | Suecia | Åloppe | — |
| Sandwich Bay Bird Observatory | Reino Unido | — | — |
| Sanglok Observatory | Tayikistán | Khatlon Province | — |
| Sant Celoni observatory | España | San Celoni | — |
| Sant Gervasi Observatory | España | Vallvidrera, el Tibidabo i les Planes | — |
| Santa Coloma de Gramenet observatory | Italia | Siena | — |
| Santa Mama observatory | Italia | Arezzo | — |
| Santa Maria Magnetic Observatory | Portugal | Azores | 196 m |
| Santa Pola Observatory | España | Santa Pola | — |
| Santana Observatory | Estados Unidos | California | — |
| Santiago-San Bernardo observatory | Chile | Santiago de Chile | — |
| Santorini Volcano Study and Monitoring Institute | Grecia | — | — |
 Sapporo Astronomical Observatory | Japón | Chūō-ku | — |
 Sapporo Science Center | Japón | Sapporo | — |
 SARA-CT | Chile | Región de Coquimbo | 2.123 m |
 SARA-KP | Estados Unidos | Arizona | 2.133 m |
| Sassoeiros observatory | Portugal | Freguesia de Carcavelos | — |
| SATech-01 | — | — | — |
 Satellite Communications Center, Medvezh'i Ozyora | Rusia | Dolgoye Lyodovo village | 200 m |
| SATINO Remote observatory | Francia | Saint-Michel-l'Observatoire | — |
 Satélite de Sondeo de Exoplanetas en Tránsito | Estados Unidos | — | — |
| Sauro Donati Astronomical Observatory | Italia | Lucca | — |
| Savelli Observatory | Italia | Savelli | — |
| Savigny-le-Temple observatory | Francia | Savigny-le-Temple | — |
 Sayan Solar Observatory | Rusia | Mondy | — |
| Sayre Observatory | Estados Unidos | South Bethlehem | — |
| Schaerding observatory | Austria | 178243 Schaerding | — |
| Schafmatt Observatory | Suiza | Oltingen | 819 m |
| Schela Observatory | Rumania | — | — |
| Scheuren Observatory | Alemania | Alemania | 200 m |
| Schmelz Observatory | Francia | Schmelz | — |
| Schmidt Observatory System | — | — | — |
| Schonfeld observatory | Alemania | Schönfeld-Weißig | — |
| School for Inclement Weather | — | — | — |
| Schothorst Public Observatory | Países Bajos | Amersfoort | — |
| Schuelerlabor Astronomie St. 7 observatory | Alemania | Wuppertal | — |
| Schwerin observatory | Alemania | Schwerin | — |
| Scott Base Magnetic Observatory | — | — | — |
| Scott Observatory | Estados Unidos | Fitchburg | — |
 Scottish Dark Sky Observatory | Reino Unido | East Ayrshire | — |
| Seagrave Memorial Observatory | Estados Unidos | — | — |