| Manggeut Observatory | Corea del Sur | Bogil-myeon | — |
| Manhay Magnetic Observatory | Bélgica | — | — |
 Mannheim Observatory | Alemania | Mannheim | — |
 Mannou Observatory | Japón | — | — |
| MAO observatory | Marruecos | Chaabet Ain Laqsab | — |
| MAP observatory | Chile | San Pedro de Atacama | — |
| Marana observatory | Italia | Crespadoro | — |
| Marburg observatory | Alemania | Marburgo | — |
| Margaret M. Jacoby Observatory | Estados Unidos | Rhode Island | — |
| Margherita Hack observatory | Italia | Florencia | — |
| Marienburg observatory | Alemania | Hildesheim | — |
| Marina di Cerveteri observatory | Italia | Marina di Cerveteri | — |
| Marina Sky observatory | España | Nerpio | — |
 Marina Towers Observatory | Reino Unido | Swansea | — |
| Marine Observatory | Alemania | Hamburgo | — |
| Maritime Alps Observatory | Italia | Cuneo | — |
| Mark Evans Observatory | Estados Unidos | Bloomington | — |
| Mark Slade Remote Observatory | Estados Unidos | Virginia | — |
| Marl observatory | Alemania | Marl | — |
| Marratxi observatory | España | Talaiot de Es Cavulls | — |
 Marseille marigraph | Francia | 7th arrondissement of Marseille | — |
| Marsfield observatory | Australia | Marsfield | — |
| Martesana Observatory | Italia | Cassina de' Pecchi | — |
| Martigues observatory | Francia | Martigues | — |
| Martin de Vivies - Amsterdam Island Magnetic Observatory | French Southern and Antarctic Lands | — | 50 m |
| Martin Observatory | Estados Unidos | Blacksburg | — |
| Martin S. Kraar observatory | Israel | Rejovot | — |
| Martz Observatory | Estados Unidos | Nueva York | — |
| Marxuquera observatory | España | Marchuquera | — |
| Maryland Space Grant Consortium Observatory | Estados Unidos | Baltimore | — |
| Masia Cal Maciarol Modul 2 | España | Ager | — |
| Masia Cal Maciarol Modul 8 | España | Ager | — |
| Masquefa Observatory | España | Masquefa | — |
| MASTER-IAC Observatory, Tenerife | España | Parque Nacional del Teide | — |
| MASTER-II observatory | Rusia | Shirotnaya Street | — |
| MASTER-II observatory | Rusia | Buryatia | — |
| MASTER-II Observatory | Rusia | Kislovodsk | — |
| MASTER-OAFA Observatory | Argentina | Provincia de San Juan | — |
| MASTER-Tavrida observatory | Ucrania | Nauchnyi | — |
| Matan Observatory | — | — | — |
| Mataro observatory | España | Mataró | — |
| Matera Laser Ranging observatory | Italia | Matera | — |
| Matosinhos Observatory | Portugal | Lanheses | — |
 Matsushiro Seismological Observatory | Japón | Nagano | — |
| Mauguio observatory | Francia | Mauguio | — |
 Mauna Loa Observatory | Estados Unidos | Hawaii County | 3.397 m |
 Mauna Loa Solar Observatory | Estados Unidos | — | — |
| Mauren Valley observatory | Alemania | Holzgerlingen | — |