| Iwaki observatory | Iwaki | — |
| Japan Public observatory Society | — | — |
| JCPM Hamatonbetsu Station | Hamatonbetsu | — |
| JCPM Kimachi Station | Shimizu | — |
| JCPM Oi Station | Shizuoka | — |
| JCPM Sakura Station | Sakura | — |
| JCPM Sapporo Station | Sapporo | — |
| JCPM Tone Station | Shimizu | — |
| Kagiya observatory | Región de Tokai | — |
| Kahoku observatory | Kahoku | — |
| Kajigamori Observatory | — | — |
| Kakioka Magnetic Observatory | Ishioka | — |
| Kamihoriguchi observatory | Ōta | — |
 Kamioka Gravitational wave detector | — | 414 m |
| Kamioka Liquid Scintillator Antineutrino Detector | Hida | — |
| Kamioka Observatory | Hida | — |
| Kani observatory | Kani | — |
| Kanoya Magnetic Observatory | — | — |
| Karasuyama observatory | Nasukarasuyama | — |
| Katori observatory | Distrito de Katori | — |
| Kawabe Observatory | Distrito de Hidaka | — |
| Kawachi observatory | Kawachi | — |
 Kawaguchi Science Museum | Kami-Aoki | — |
| Kawane observatory | Kawane | — |
| Kazo Miraikan | Kazo | — |
 Kihoku Astronomical Museum | Kanoya | — |
| Kiyosato observatory | Kiyosato | — |
| Kiyose and Mizuho observatory | Kiyose | — |
| Kochi observatory | Kōchi | — |
| Kogota observatory | Kogota | — |
| Konan observatory | Konan | — |
 Koyama Astronomical Observatory, Kyoto Sangyo University | Kita-ku | — |
| Kuma Kogen Astronomical Observatory | Kumakōgen | 609 m |
| Kumagaya City Cultural Center Planetarium Hall | Kumagaya | — |
| Kumamoto | prefectura de Kumamoto | — |
 Kumamoto Civil Astronomical Observatory | Minami-ku | — |
 Kurashiki Observatory | Kurashiki | — |
| Kure City Kamagari Astronomical Observatory | Kure | — |
| Kurihara observatory | Kurihara | — |
| Kurohone observatory | Kurohone | — |
| Kurume City Observatory | prefectura de Fukuoka | — |
| Kushiro observatory | Kushiro | — |
 Kwasan | Yamashina-ku | — |
| LiteBIRD | — | — |
| Machida observatory | Machida | — |
 Mannou Observatory | — | — |
 Matsushiro Seismological Observatory | Nagano | — |
| Memambetsu Magnetic Observatory | — | — |