| Lunar Cafe Observatory | Estados Unidos | Míchigan | — |
| Lunar Infrastructure for Exploration | — | — | — |
| Lunping Magnetic Observatory | República de China | — | — |
| Luscherz Observatory | Suiza | Lüscherz | — |
| Lviv Magnetic Observatory | Ucrania | — | — |
 Lviv Polytechnic Observatory | Ucrania | Leópolis | — |
| Lvye Observatory | República Popular China | Huqiu | — |
| Lycksele Magnetic Observatory | Suecia | — | — |
 Lynx X-ray Surveyor | — | — | — |
| Lågskär bird observatory | Finlandia | Lemland | — |
 Lübeck observatory | Alemania | Lübeck | 10 m |
| M. R. Stefanik Observatory and Planetarium | Eslovaquia | Hlohovec | — |
| M57 Observatory | Italia | Saltrio | — |
 Macalester College Observatory | Estados Unidos | Saint Paul | — |
| Macclesfield observatory | Reino Unido | Macclesfield | — |
| Mace Head Atmospheric Research Station | Irlanda | Halfmace | — |
| Machida observatory | Japón | Machida | — |
| Mackay observatory | España | San Cristóbal de La Laguna | — |
| Macnairston Observatory | Reino Unido | South Ayrshire | — |
| Macquarie Island Magnetic Observatory | Australia | Isla Macquarie | — |
 Madhava Observatory | India | — | — |
| Madison-YRS observatory | Estados Unidos | Wisconsin | — |
 Madras Observatory | India | Chennai | — |
| Magadan Magnetic Observatory | Rusia | — | — |
| Magellan observatory | Australia | Lake Bathurst | — |
| Magroforte observatory | Italia | Magroforte Inferiore | — |
 Maiaky Observatory | Ucrania | Óblast de Odesa | 27 m |
| Maidanak Observatory | Uzbekistán | Provincia de Kashkadar | 2.650 m |
| Maidbronn observatory | Alemania | Rimpar | — |
| Maiden Lane observatory | Estados Unidos | Bainbridge Island | — |
| Maidenhead observatory | Reino Unido | Windsor y Maidenhead | — |
 Main Astronomical Observatory of the National Academy of Sciences of Ukraine | Ucrania | Holosiiv | 213 m |
 Main building of the Old Observatory, Leiden | Países Bajos | Leiden | — |
| Maisoncelles observatory | Francia | Saint-Martin-du-Boschet | — |
| Maisons Laffitte observatory | Francia | Maisons-Laffitte | — |
| Majadahonda observatory | España | Majadahonda | — |
| Makerstoun Observatory | Reino Unido | Scottish Borders | — |
| Makes Observatory | Francia | Saint-Louis | — |
| Malaga Observatory | España | Málaga | — |
 Malakoff Tower | Brasil | Recife | — |
| Malibert observatory | Francia | Babeau-Bouldoux | — |
| Malina River observatory | Italia | Ravosa-Magredis | — |
| Mamalluca Observatory | Chile | Plaza Vicuña Mackenna | — |
 Man Singh Observatory | India | Benarés | — |
| Manaratu AlUla | Arabia Saudí | Medina Province | — |
| Manastash Ridge Observatory | Estados Unidos | Washington | — |
| Manastash Ridge Radar | Estados Unidos | Washington | — |
| Mandi observatory | Italia | Údine | — |