| Maury Lewin Observatory | Estados Unidos | Glendora | — |
| Mawson Magnetic Observatory | — | — | — |
| Max Valier Sat | Italia | — | — |
| Maynard F. Jordan Observatory | Estados Unidos | Maine | — |
| Mazagon Huelva observatory | España | Mazagán | — |
| Mazariegos observatory | España | — | — |
| Mbour Magnetic Observatory | Senegal | — | — |
| McDonald Observatory-LCO ELP | Estados Unidos | Fort Davis | — |
| McDonald Observatory-LCO ELP Aqawan A #1 | Estados Unidos | Fort Davis | — |
| McDonald Observatory-LCO ELP B | Estados Unidos | Fort Davis | — |
 McKim Observatory | Estados Unidos | Greencastle | — |
| McMillin Observatory | Estados Unidos | Ohio | — |
 Mead Observatory | Estados Unidos | Georgia | — |
| Meanook Magnetic Observatory | Canadá | — | — |
| Medina Dome observatory | Estados Unidos | Medina | — |
| MeerLICHT | Sudáfrica | Karoo Hoogland | — |
| Mehalso Observatory | Estados Unidos | Pensilvania | — |
| Mehr Bushehr Observatory | Irán | — | — |
 Melbourne Observatory | Australia | Victoria | — |
| Melezhy Astrophoto Observatory | Rusia | Óblast de Vladímir | — |
| Melton Memorial Observatory | Estados Unidos | Carolina del Sur | — |
| Memambetsu Magnetic Observatory | Japón | — | — |
 Menara Kuala Lumpur | Malasia | Kuala Lumpur | — |
| Mendel Observatory | Estados Unidos | North Andover | — |
| Mendenhall Observatory | Estados Unidos | Oklahoma | — |
| Menke Observatory | Estados Unidos | Iowa | — |
| Merate astronomical observatory | Italia | Merate | — |
| Meridian Pillars, Brisbane House | Reino Unido | North Ayrshire | — |
| Meridian Sitings | Reino Unido | Tullyard | — |
| Merignac observatory | Francia | Mérignac | — |
| Merlette observatory | Francia | Altos Alpes | — |
| Meschede observatory | Alemania | Meschede | — |
| Meteorological observatory Praha-Libuš | República Checa | Kamýk | — |
 Meteorological station Milešovka | República Checa | Velemín | — |
 Meteorologisches Observatorium Potsdam | Alemania | Potsdam | — |
| Metsahovi Optical Telescope observatory | Finlandia | Kirkkonummi | — |
| Metsala observatory | Finlandia | Espoo | — |
| Metsähovi Geodetic Research Station | Finlandia | Kirkkonummi | — |
 Metsähovi Radio Observatory | Finlandia | Kirkkonummi | — |
| Meyer–Womble Observatory | Estados Unidos | Colorado | — |
| Michiel Coignet Observatorium | Bélgica | — | — |
 Michigan State University Observatory | Estados Unidos | — | — |
 Michigan State University Observatory | Estados Unidos | Míchigan | — |
 Michigan-Dartmouth-MIT Observatory | Estados Unidos | Tucson | — |
| Mickleover observatory | Reino Unido | Mickleover | — |
| Micro Palomar observatory | Francia | Reilhanette | — |
 MicroObservatory | Estados Unidos | — | — |
| Microvariability and Oscillations Of STars | — | — | — |