| 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 | — |
| U green frog observatory | República Checa | Ondřejov | — |
| U. of Minn. Infrared Observatory | Estados Unidos | Mount Lemmon | — |