| Mid Atlantic Star Party | Estados Unidos | Carolina del Norte | — |
| Middlebury College Observatory | Estados Unidos | Vermont | — |
| Middlesex School Observatory | Estados Unidos | Concord | — |
| Midway Island Magnetic Observatory | Estados Unidos | — | — |
 Mikhailo Lomonosov | — | — | — |
 Milford Sound Discovery Centre and Underwater Observatory | Nueva Zelanda | Distrito Southland | — |
| Mill Creek observatory | Estados Unidos | Chenoweth | — |
| Millau observatory | Francia | Millau | — |
| Miller Observatory | Estados Unidos | Carolina del Norte | — |
 Millimeter Anisotropy eXperiment IMaging Array | Estados Unidos | — | — |
 Millstone Hill Observatory | Estados Unidos | Massachusetts | — |
| Millwood Observatory | Estados Unidos | Center Point (Texas) | — |
 Milmaru Observatory | Corea del Sur | — | — |
 Milwaukee Astronomical Society Observatory | Estados Unidos | New Berlin | — |
| Minami-Oda Observatory | Japón | Minami | — |
| Minamiaso Luna Observatory | Japón | Minamiaso | — |
 MINERνA | Estados Unidos | — | — |
| MiniBooNE | Estados Unidos | — | — |
| Minnetonka observatory | Estados Unidos | Minnetonka | — |
| MINOS+ | Estados Unidos | — | — |
 Misato Observatory | Japón | Distrito de Kaisō | — |
| Mishima observatory | Japón | Susono | — |
| Missouri City observatory | Estados Unidos | Missouri City | — |
 Mitzpe Hadar | Israel | — | 263 m |
 Mitzpe Revivim | Israel | Concejo Regional Ramat HaNéguev | — |
| Miwa observatory | Japón | Manza Onsen Ski Resort | — |
| Miyaki-Argenteus observatory | Japón | Miyaki | — |
| Miyasaka Observatory | Japón | Miyasaka | — |
| Mizusawa VERA Observatory | Japón | — | — |
| Moana observatory | Francia | Puna'auia observatory | — |
| Mobil Astronomical Robotics Genon Observatory | Rusia | Nauchnyi | 610 m |
| Moka Observatory | Estados Unidos | Benson | — |
| Molino de Viento con observatorio | España | — | — |
 Molėtai Astronomical Observatory | Lituania | Distrito de Molėtai | 200 m |
| MOMA observatory | España | Oviedo | — |
 Mon Plaisir Tower | Polonia | Świeradów-Zdrój | — |
| MONET/South | Sudáfrica | Karoo Hoogland | — |
 Mont d'Arbois astronomical observatory | Francia | Saint-Gervais-les-Bains | — |
 Mont Mégantic Observatory | Canadá | La Patrie | 1.105 m |
| Montagut observatory | España | Tarrasa | — |
| Montana Blanca observatory | España | Tías | — |
| Montarrenti observatory | Italia | Sovicille | — |
| Montcabrer Osservatory | España | Cabrils | 120 m |
| Monte Argentario observatory | Italia | Monte Argentario | — |
| Monte Autore observatory | Italia | Monte Autore | — |
| Monte Baldo observatory | Italia | Ferrara di Monte Baldo | 1.220 m |
| Monte Bianco obsertatory | Italia | Chamonix-Mont-Blanc | 4.807 m |
| Monte Deva observatory | España | Monte Deva | — |