I
| Name | Established | Location |
| Indian Astronomical Observatory | 2001 | Hanle, Jammu and Kashmir, India |
| Instituto de Astrofísica de Canarias | 1975 | Canary Islands, Spain |
| Institut de radioastronomie millimétrique | 1979 | Spain, France and Germany |
| INTEGRAL | 2002 | Highly eccentric Earth orbit |
| Infrared Astronomical Satellite (IRAS) (defunct) | January–November 1983 | Earth orbit |
| Infrared Space Observatory (ISO) (defunct) | 1995–1998 | Earth orbit |
| Innsbruck Observatory | Innsbruck, Austria | |
| IRAM 30m telescope | 1984 | Pico Veleta, Spain |
| IRAM Plateau de Bure interferometer | 19?? | Plateau de Bure, France |
| Isaac Newton Group of Telescopes | 1979 | La Palma, Spain |
| Iso-Heikkilä Observatory (defunct) | 1935 (only amateur use since 1972) | Turku, Finland |
| Istanbul observatory of Taqi al-Din | 1577 | Istanbul, Turkey |
Read more about this topic: List Of Astronomical Observatories