 The Virtual Telescope Project | Italia | Ceccano | — |
 The Voeikov Main Geophysical Observatory | Rusia | San Petersburgo | — |
| THEMIS Ground-based Observatory | — | — | — |
 Theodor Jacobsen Observatory | Estados Unidos | Seattle | — |
| THESEUS | — | — | — |
| Thomas G. Cupillari Observatory | Estados Unidos | Fleetville | — |
 Thomas King Observatory | Nueva Zelanda | Wellington City | — |
| Thomas More College Observatory | Estados Unidos | Kentucky | — |
| Thompson Observatory | Estados Unidos | Wisconsin | — |
| Thompsonville Observatory | Estados Unidos | Thompsonville | — |
| Thornlands observatory | Australia | Thornlands | — |
| Thornton observatory | Estados Unidos | Thornton | — |
| Three Buttes observatory | Estados Unidos | Saddlebrooke | — |
| Three College Observatory | Estados Unidos | Carolina del Norte | — |
 Tiana observatory | España | Tiana | — |
| TianQin | República Popular China | — | — |
 Tican Observatory | Croacia | Višnjan | — |
| Tigh Speuran Observatory | Estados Unidos | Framingham | — |
| Tihany Magnetic Observatory | Hungría | — | — |
| Tiki observatory | Francia | Puna'auia observatory | — |
 Timau National Observatory | Indonesia | Kupang | 1.347 m |
| Timberlake Observatory | Estados Unidos | Reston | — |
| Timberland Observatory | Estados Unidos | Alabama | — |
| Timberline observatory | Estados Unidos | Winterset | — |
 Timișoara Astronomical Observatory | Rumania | Timișoara | — |
| Tivoli Observatory | Namibia | Jardines de Tivoli | — |
| Tixie Bay Magnetic Observatory | Rusia | — | — |
| TLC Observatory | Italia | Bolonia | — |
| Toby Point Observatory | Estados Unidos | Narragansett | — |
 Tochigi Science Museum | Japón | Utsunomiya | — |
| Tocororo Observatory | España | Arquillinos | — |
| Tokai observatory | Japón | Tōkai | — |
| Tokigawa Town Star and Green Creation Center | Japón | Tokigawa | — |
| Tokushima observatory | Japón | Tokushima | — |
| Tokyo Astronomical Observatory (before 1938) | Japón | Tokio | — |
| Tokyo-Asahikawa observatory | Japón | Tokio | — |
| Tokyo-Dodaira observatory | Japón | Tokio | — |
| Tokyo-Mitaka observatory | Japón | Tokio | — |
| Tokyo-Norikura observatory | Japón | Tokio | — |
| Tokyo-Okayama observatory | Japón | Tokio | — |
| Tolar astronomical observatory | Argentina | Tolar Grande | — |
| Toma observatory | Japón | Toma | — |
| Tomsk State University Observatory | Rusia | Tomsk | 137 m |
 Torre de la Babotte | Francia | Montpellier | — |
 Torre Einstein | Alemania | Potsdam-Süd | — |
| Torre Villa Boffi observatory | Italia | Giussano | — |
| Torreaguila observatory | España | Barbaño | — |
| Torredembarra observatory | España | Torredembarra | — |