 Table Mountain Observatory | Estados Unidos | Big Pines | 2.285 m |
| Tacande Observatory | Canarias | El Paso | — |
| Tachibana Observatory | Japón | Miyakonojō | — |
| Taeduk Radio Astronomy Observatory | Corea del Sur | — | 109 m |
| Tajimi observatory | Japón | Tajimi | — |
| Takanezawa observatory | Japón | Takanezawa | — |
 Takapuna observatory | Nueva Zelanda | Auckland Region | — |
 Talcott house | Países Bajos | Leiden | — |
| Talcott Mountain Science Center | Estados Unidos | Connecticut | — |
 Tallinn Observatory | Estonia | Tallinn | 57,8 m |
| Tamanrasset Magnetic Observatory | Argelia | Tamanrasset | — |
| Tamaris Observatory | Francia | La Seyne-sur-Mer | — |
 Tamke-Allan Observatory | Estados Unidos | Tennessee | — |
| Tamworth Observatory | Australia | Tamworth | — |
| Tangra observatory | Australia | St. Clair | — |
| Tara Observatory | Estados Unidos | Pontotoc | — |
| Tarbatness Observatory | Reino Unido | Highland | — |
| Tarleton State University Observatory | Estados Unidos | Stephenville | — |
| Tarzana observatory | Estados Unidos | Los Ángeles | — |
| Tashkent observatory | Uzbekistán | Taskent | — |
| Tataka observatory | República de China | Sinyi Township | — |
| Tatuoca Geomagnetic Observatory | Brasil | Pará | — |
| Taunton observatory | Estados Unidos | Taunton | — |
| Taurus Australis Observatory | Brasil | Taguatinga | — |
 Taurus Hill observatory | Finlandia | Varkaus | — |
 Taurus-1 Observatory | Bielorrusia | Provincia de Vítebsk | — |
 Tavolaia astronomical observatory | Italia | Santa Maria a Monte | — |
| Taylor Range Observatory | Australia | Brisbane | — |
 Tbilisi Observatory | Georgia | Tiflis | — |
| Tebbutt Observatory | Estados Unidos | Edgewood | — |
| TechDome observatory | Estados Unidos | Weed Lookout Tower | — |
| Telescope Live | España | Cortijo de las Minas | — |
| Telescope Live observatory | Chile | Río Hurtado | — |
 Telescopio de modulación de rayos X duros | República Popular China | — | — |
 Telescopio espacial Hubble | — | — | — |
 Telescopio espacial James Webb | — | — | — |
 Telescopio espacial Nancy Grace Roman | Estados Unidos | — | — |
 Telescopio Espacial Solar Galileo | Brasil | — | — |
 Telescopio espacial Spitzer | — | — | — |
| Telescopio extremadamente pequeño de kilogrado | Estados Unidos | — | — |
 Telescopio Fermi | — | — | — |
 Telescopio Gemini Frederic C. Gillet | Estados Unidos | Hawaii County | 4.213 m |
 Telescopio Hooker | Estados Unidos | — | — |
 Telescopio infrarrojo de NASA | Estados Unidos | Hawaii County | — |
 Telescopio Jacobus Kapteyn | España | — | 2.369 m |
| Telescopio subacuático de neutrinos de Baikal | Rusia | Baikal | — |
 Telescopio Subaru | Estados Unidos | Hawái | 4.139 m |
| Telescopio XO | Estados Unidos | Hawái | — |
 Telescopio óptico infrarrojo | Estados Unidos | condado de Santa Cruz | — |
| Telford observatory | Reino Unido | Great Dawley | — |
| Teluk Kemang Observatory | Malasia | Negeri Sembilan | — |
| Temecula observatory | Estados Unidos | Temecula | — |
| Ten Acre Observatory | Estados Unidos | Oklahoma | — |
| Tenagra II Observatory | Estados Unidos | Arizona | 1.312 m |
| Tenerife Observatory-LCO A, Tenerife | España | Parque Nacional del Teide | — |
| Tenerife Observatory-LCO B, Tenerife | España | Parque Nacional del Teide | — |
| Tenerife-LCO Aqawan A #1 | España | Parque Nacional del Teide | — |
| Tenerife-LCO Aqawan A #2 | España | Parque Nacional del Teide | — |
| Teplice Observatory | República Checa | Teplice | — |
| Terminus observatory | España | Oleiros | — |
| Terrassa observatory | España | Tarrasa | — |
| Terre Haute observatory | Estados Unidos | Terre Haute | — |
| Terres Blanches observatory | Francia | canton of Reillanne | — |
 Terrestrial Planet Finder | — | — | — |
| Tesla observatory | España | Valdemorillo | — |
| Tetzacualco | México | — | — |
| Texas A&M Astronomical Observatory | Estados Unidos | Texas | — |
| Texas A&M Physics observatory | Estados Unidos | College Station | — |
| Texas Star Party, Inc. | Estados Unidos | — | — |
 Teylers astronomical observatory | Países Bajos | Haarlem | — |
 Thai National Observatory | Tailandia | — | 2.457 m |
 The astrodome of the Lyceum in Eger | Hungría | Eger | — |
| The Cottage Observatory | Estados Unidos | Altoona | — |
| The E and B Experiment | — | — | — |
 The Equatorial Observatory, Penllergare | Reino Unido | Penllergaer | 79,7 m |
 The Heights Observatory | Australia | South Australia | — |
 The Observatory | Reino Unido | Falmouth | — |
 The Octagon at Amherst College | Estados Unidos | Massachusetts | — |
| The Ranch Observatory | Estados Unidos | New Mexico | — |
 The Space Observatory (Observatoire de l'Espace) | Francia | — | — |
| The Spaceguard Center observatory | Gales | Knighton | — |
| The Studios Observatory | Reino Unido | Grantham | — |
 The Tower of the Sun | Italia | Brembate di Sopra | — |
 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 | — |
| Tortona observatory | Italia | Tortona | — |
| Tortugas Mountain Observatory | Estados Unidos | New Mexico | — |
| Torus Observatory | Estados Unidos | Buckley | — |
| Tosa observatory | Japón | Tosa | — |
 Townsend Observatory | Nueva Zelanda | Christchurch City | — |
 Toyama City Observatory | Japón | Prefectura de Toyama | — |
| Toyama observatory | Japón | Toyama | — |
| Toyonaka observatory | Japón | Toyonaka | — |
| Toyota observatory | Japón | Toyota | — |
| Transit Dreams Observatory, Campobello | Estados Unidos | Campobello | — |
| Transit of Venus Observation Site | Nueva Zelanda | Selwyn District | — |
 Transition Region and Coronal Explorer | Estados Unidos | — | — |
| TRAPPIST-North observatory | Marruecos | Oukaïmeden | — |
| Trebur observatory | Alemania | Trebur | — |
| Trelew Magnetic Observatory | Argentina | — | — |
| Trent Astronomical Observatory | Reino Unido | City of Nottingham | — |
| Tres Cantos Observatory | España | Madrid | — |
| Tristan da Cunha Magnetic Observatory | Reino Unido | — | 42 m |
 Trivandrum Observatory | India | — | — |
| Troodos Observatory | Chipre | Limasol | 1.250 m |
| TRT-NEO observatory | Tailandia | Chang Khoeng | — |
| Trumbull Observatory | Estados Unidos | Trumbull | — |
| TRZ observatory | España | Caseta de les Mallades | — |
 Tsinghua University Observatory | República Popular China | Pekín | — |
| Tsingtao field station | República de China | Islas Paracel | — |
| Tsingtao Observatory | República Popular China | A method for resolving spectra shift in the urban air quality monitoring system (DOAS) | — |
| Tsumeb Magnetic Observatory | Namibia | — | — |
| Tucci Observatory | España | Martos | — |
| Tucson Magnetic Observatory | Estados Unidos | — | — |
| Tucson-Winterhaven observatory | Estados Unidos | Tucson | — |
| Tumamoc Hill observatory | Estados Unidos | Tucson | — |
 Tuorla Observatory | Finlandia | Kaarina | 60 m |
| Turin Observatory | Italia | Turín | — |
| Turin observatory (before 1913) | Italia | Turín | — |
| Turitea observatory | Nueva Zelanda | Turytsia | — |
 Turnov Observatory | República Checa | Turnov | — |
| Turtle Star Observatory | Alemania | Mülheim an der Ruhr | 45 m |
| Tuscolana astronomy association | Italia | Rocca di Papa | — |
 Tweelingen public observatory | Países Bajos | Nissewaard | — |
| Tweenhills Observatory | Reino Unido | Hartpury | — |
| Tyrone observatory | Estados Unidos | condado de Grant | — |
| Tzec Maun observatory | Australia | Moorook | — |
| Tzec Maun Observatory | Estados Unidos | Mayhill | — |
| Tzec Maun observatory (before 2010) | Australia | Pingelly | — |
 Tähtikallio Observatory & Education Center | Finlandia | Orimattila | — |
 Třebíč observatory | República Checa | Horka Domky | — |