 John Payson Williston Observatory | Estados Unidos | — | — |
| Joint observatory for cometary research | Estados Unidos | Socorro | — |
| Jonathan B. Postel Observatory | Italia | Promiod | — |
 Jones Observatory | Estados Unidos | Chattanooga | — |
| Jonsrud Viewpoint | Canadá | — | — |
| Jornada Observatory | Estados Unidos | Las Cruces | — |
| Josef Bresser Observatory | Alemania | Distrito de Borken | — |
| JPL SynTrack Robotic Telescope | Estados Unidos | Alder Springs | — |
| JPL SynTrack Robotic Telescope 2 | Estados Unidos | Alder Springs | — |
| JPL SynTrack Robotic Telescope 3 | Estados Unidos | Flagstaff | — |
| Judson B. Coit Observatory | Estados Unidos | — | — |
| Jura-Sternwarte Grenchen | Suiza | Grenchen | — |
 Ka-Dar Observatory | Rusia | Karacháyevo-Cherkesia | 2.060 m |
 Ka-Dar Observatory | Rusia | Kuzminskoye | 180 m |
| Kachina observatory | Estados Unidos | Kachina Village | — |
 Kadaň Observatory | República Checa | Kadaň | 399 m |
| Kagiya observatory | Japón | Región de Tokai | — |
| Kahoku observatory | Japón | Kahoku | — |
| Kairos Observatory | Rusia | Rostov | — |
| Kajigamori Observatory | Japón | — | — |
| Kakadu Magnetic Observatory | Australia | Northern Territory | — |
| Kakioka Magnetic Observatory | Japón | Ishioka | — |
| Kalamazoo observatory | Estados Unidos | Municipio de Kalamazoo | — |
 Kalyazin Radio Astronomy Observatory | Rusia | Kaliazin | 86 m |
| Kambah observatory | Australia | Kambah | — |
 Kamenskoe Plateau Observatory | Kazajistán | Alma Ata | 1.450 m |
| Kamihoriguchi observatory | Japón | Ōta | — |
 Kamioka Gravitational wave detector | Japón | — | 414 m |
| Kamioka Liquid Scintillator Antineutrino Detector | Japón | Hida | — |
| Kamioka Observatory | Japón | Hida | — |
| Kanab observatory | Estados Unidos | Cannabis sativa | — |
| Kandilli Magnetic Observatory | Turquía | — | — |
 Kandilli Observatory | Turquía | Estambul | — |
| Kangwon National University Observatory | Corea del Sur | Gangnam-dong | — |
| Kangwon Science High School observatory | Corea del Sur | Wonju | — |
| Kani observatory | Japón | Kani | — |
| Kanoya Magnetic Observatory | Japón | — | — |
 Kanzelhoehe Solar Observatory | Austria | Arriach | — |
| Kappa Crucis observatory | Argentina | Córdoba | — |
| Karasuyama observatory | Japón | Nasukarasuyama | — |
| Karlovy Vary Observatory | República Checa | Karlovy Vary | — |
 Karlsruhe Observatory | Alemania | Karlsruhe | 125 m |
| Karrenkneul observatory | Alemania | Calden | — |
| Karthala Volcano Observatory | Comoras | — | — |
 Kasin Observatory | Irán | — | — |
| Katori observatory | Japón | Distrito de Katori | — |
 KATRIN | Alemania | Karlsruhe | — |
| Katy Observatory | Estados Unidos | Texas | — |