| IAA-AI Atacama observatory | Chile | San Pedro de Atacama | — |
| Iași Astronomical Observatory | Rumania | Condado de Iași | — |
| Ibis observatory | Estados Unidos | Manhattan | — |
 IceCube | — | Región del Tratado Antártico | — |
| Iga-Ueno observatory | Japón | Tsuge Station | — |
| Iitate Planetary Radio Telescope | Japón | Prefectura de Fukushima | — |
| Ilmala observatory | Finlandia | Helsinki | — |
| Iluro observatory | España | Mataró | — |
 Imaging X-ray Polarimetry Explorer | Estados Unidos | — | — |
| Immanuel Kant observatory | Alemania | Limbach-Oberfrohna | — |
| Inagawa Observatory | Japón | Distrito de Kawabe | — |
| Inastars observatory | Alemania | Potsdam | — |
| Inastars Observatory Potsdam ( before 2006) | Imperio alemán | Potsdam | — |
 Indian Astronomical Observatory (IAO) | India | Hanle | 4.500 m |
| Indian Hill North Observatory | Estados Unidos | Huntsburg | — |
| Ingenio observatory | España | Ingenio | — |
| Ingle Farm observatory | Australia | Ingle Farm | — |
 INPE, Cachoeira Paulista | Brasil | Cachoeira Paulista | — |
| Insperity observatory | Estados Unidos | Humble | — |
 Instalación de Calefacción Ionosférica Sura | Rusia | — | — |
| Institute for Research in Volcanology and Risk Assessment | Portugal | — | — |
 Institute of Avian Research "Vogelwarte Helgoland" | Alemania | Wilhelmshaven | — |
| Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University | Japón | Shimabara | — |
 Instituto Argentino de Radioastronomía (IAR) | Argentina | Berazategui | 20 m |
 Instituto de Astrofísica de Potsdam | Alemania | Potsdam | — |
| Instituto de Astronomía y Meteorología | México | — | — |
 Instituto de Cálculo Astronómico | Alemania | Heidelberg | — |
| Instituto de Investigaciones Astronómicas de Pisgah | Estados Unidos | Carolina del Norte | — |
 Instituto de Radioastronomía Milimétrica | Francia | Grenoble | — |
 Instituto de Vulcanología y Sismología | Rusia | Petropávlovsk-Kamchatski | — |
| Instituto Geofísico de la Escuela Politécnica Nacional | Ecuador | Quito | — |
 Instituto Nacional de Sismología, Vulcanología, Meteorología e Hidrología de Guatemala | Guatemala | — | — |
| Instituto Nicaragüense de Estudios Territoriales | Nicaragua | Managua | — |
| Instituto Volcanológico de Canarias, S.A. | España | San Cristóbal de La Laguna | — |
 INTEGRAL | — | — | — |
 Inter-University Centre for Astronomy and Astrophysics | India | Pune | — |
 Interface Region Imaging Spectrograph | — | — | — |
| International Astronomy Center | Emiratos Árabes Unidos | — | — |
 International Ultraviolet Explorer | — | — | — |
| Iota Scorpii Observatory | Italia | La Spezia | — |
| Iowa Robotic Observatory | Estados Unidos | Elgin | — |
| Iqaluit Magnetic Observatory | Canadá | — | — |
 Iranian National Observatory | Irán | Qamsar District | 3.600 m |
| Irkutsk Magnetic Observatory | Rusia | — | — |
| Irmtraut observatory | Alemania | Irmtraut | — |
| Ironwood observatory | Estados Unidos | Haleiwa | — |
| Ironwood Remote observatory | Estados Unidos | Honolulu County | — |
| Irvine–Michigan–Brookhaven | Estados Unidos | — | — |
| Irydeo Observatory | España | Camarma de Esteruelas | — |
 Isaac Newton Group of Telescopes | España | — | — |
| Isarwinkel observatory | Alemania | Bad Tölz | — |
 Ishigakijima Astronomical Observatory | Japón | Ishigaki | 200 m |
| Ishiki observatory | Japón | Ishiki | — |
| Isla de Pascua Mataveri (Easter Island) Magnetic Observatory | Chile | — | — |
| Isle of Man observatory | Reino Unido | Foxdale | — |
 Isle of May Bird Observatory | Reino Unido | Fife | — |
| ISON-Blagoveschensk observatory | Rusia | Blagovéshchensk | — |
| ISON-Byurakan Observatory | Armenia | Byurakan | — |
| ISON-Castelgrande Observatory | Italia | Castelgrande | — |
| ISON-Hureltogoot Observatory | Mongolia | Ulán Bator | — |
| ISON-NM Observatory | Estados Unidos | Mayhill | 2.217 m |
| ISON-SSO Observatory | Australia | Bugaldie | — |
| ISON-Terskol observatory | Rusia | Terskol | — |
| ISON-Uzhhorod Observatory | Ucrania | Óblast de Transcarpacia | — |
| Istituto Nazionale di Geofisica e Vulcanologica - Sezione di Catania | Italia | Catania | — |
| ITelescope Observatory | España | Nerpio | — |
| ITelescope SRO Observatory | Estados Unidos | Alder Springs | — |
| Iturrieta observatory | España | Iturrieta | — |
| Ivywood observatory | Estados Unidos | Edmond | — |
| Iwaki observatory | Japón | Iwaki | — |
| Iznik Magnetic Observatory | Turquía | Nicea | — |
| İnönü University Astronomy Research and Application Center | Turquía | Malatya | 963 m |