| İnönü University Astronomy Research and Application Center | Turquía | Malatya | 963 m |
| J. C. Veen observatory | Estados Unidos | Municipio de Lowell | — |
| Jack C. Davis Observatory | Estados Unidos | Carson City | — |
| Jack C. Davis Observatory | Estados Unidos | Nevada | — |
| Jaen Observatory | España | Jaén | — |
| Jaipur Magnetic Observatory | India | — | — |
| Jakokoski Observatory | Finlandia | Jakokoski | — |
 James Cook Observatory | Nueva Zelanda | Gisborne | — |
 Jan Paagman Sterrenwacht | Países Bajos | Asten | — |
| Jana Heweliusza w Gdańsku astronomical observatory | Polonia | Gdansk | — |
 Janssen observatory | Francia | — | — |
 Jantar Mantar | India | Distrito de Nueva Delhi | — |
 Jantar Mantar | India | Jaipur | — |
| Japal-Rangapur observatory | India | Punjagutta | — |
| Japan Public observatory Society | Japón | — | — |
 Jardim da Luz observatory | Brasil | São Paulo | — |
| Jarnac Observatory | Estados Unidos | Vail | — |
| Jaroslav Trnka Observatory in Slaný | República Checa | Slaný | — |
| Javalambre observatory | España | La Higuera | — |
| JBL Observatory | Australia | Bathurst | — |
| JCPM Hamatonbetsu Station | Japón | Hamatonbetsu | — |
| JCPM Kimachi Station | Japón | Shimizu | — |
| JCPM Oi Station | Japón | Shizuoka | — |
| JCPM Sakura Station | Japón | Sakura | — |
| JCPM Sapporo Station | Japón | Sapporo | — |
| JCPM Tone Station | Japón | Shimizu | — |
 Jena Observatory | Alemania | Jena | — |
 Jennifer Chalsty Planetarium | Estados Unidos | Jersey City | — |
 Jerusalem Bird Observatory | Israel | Jerusalén | — |
| Jesuit astronomical observatory in Poznań | — | — | — |
 Jesuit observatory in Litoměřice | República Checa | Litoměřice-Město | — |
 Jewett Observatory | Estados Unidos | Pullman | 790 m |
| Jiama'erdeng Tianwentai observatory | República Popular China | Ngari | — |
| Jiang Nan Tian Chi observatory | República Popular China | Condado de Anji | — |
| Jiangnantianchi Observatory | República Popular China | Condado de Anji | — |
| Jim Carrigan Magnetic Observatory | Estados Unidos | — | — |
| Jimginny Observatory | Estados Unidos | Naperville | — |
 Jiufeng Seismic Station | República Popular China | Sujiatuo | — |
| Jičín Observatory | República Checa | Jičín | 270 m |
| Joan Roget observatory | España | Ager | — |
 Johann-Kern-Sternwarte Wertheim | Alemania | Wertheim am Main | 275 m |
| Johannes-Kepler observatory | Alemania | — | — |
 Johannes-Kepler-Sternwarte | Austria | Linz | — |
 Johannes-Kepler-Volkssternwarte | Austria | Thal | — |
| Johannesburg-Hartbeespoort | Sudáfrica | Johannesburgo | — |
| John Beckman observatory | España | Retamar y el Toyo | — |
| John H. Witte, Jr. Observatory | Estados Unidos | Iowa | — |
| John J. McCarthy Observatory | Estados Unidos | New Milford | — |