| Sierra Remote observatory | Auberry | — |
| Sierra Sinagua Observatory | Flagstaff | — |
| Sierra Stars observatory | Mesa Vista | — |
| Silver Spring observatory | White Oak | — |
 Sinnott Memorial Observation Station | Klamath County | 2.027 m |
| Sitka Magnetic Observatory | — | — |
 Sky's the Limit Observatory and Nature Center | Twentynine Palms | — |
| Skyledge Observatory | Killingworth | — |
 Small Astronomy Satellite 2 | — | — |
 Small Astronomy Satellite 3 | — | — |
| Smith College Observatory | Northampton | — |
| Smith Observatory and Dr. William R. Brooks House | Nueva York | — |
| Smith River Observatory | Danbury | — |
| Snohomish Hilltop Observatory | Snohomish | — |
| Snow King Observatory and Planetarium | — | — |
| Sola Fide Observatory | Condado de Mower | — |
 Solar Probe Plus | — | — |
 Solar Radiation and Climate Experiment | — | — |
 Sommers–Bausch Observatory | Colorado | — |
| Sonoita Research observatory | Sonoita | — |
 Sonoma State Observatory | California | — |
| Sonoran Skies observatory | St. David | — |
| Southeastern Arizona Bird Observatory | — | — |
| Southwest Institute for Space Research | New Mexico | — |
| SPARCS | — | — |
 SPHEREx | — | — |
| Spirit Marsh Observatory | Meire Grove | — |
| Squirrel Valley Observatory | Columbus | — |
| St. Thomas Observatory | — | — |
| Stamford observatory | Stamford | — |
| Stansbury Park Observatory Complex | Tooele | — |
| Star Cruiser Observatory | Anza | — |
| Star Ridge observatory | Weimar | — |
| Starhoo Observatory | Lakeville | — |
| Starry Knight observatory | California | — |
| Starry Night Observatory | Glidden | — |
 Stellafane | Springfield | — |
| Stennis Space Center Magnetic Observatory | — | — |
| Stephens Observatory | Maine | — |
| Stokesville Observatory | Virginia | — |
| Stone Edge observatory | El Verano | — |
| Stonegate Observatory | Ann Arbor | — |
 Stony Ridge Observatory | California | 1.730 m |
| Stull Observatory | Nueva York | — |
| Submillimeter Array, Mauna Kea (SMA) | Hilo | — |
 Submillimeter Wave Astronomy Satellite | — | — |
| Sulphur Flats observatory | Jémez Springs | — |
 Sun Dagger site | condado de San Juan | — |