 Monte Generoso Observatory | Suiza | cantón de Tesino | — |
 Monte Mario Observatory | Italia | Roma | 128 m |
 Monte Viseggi Observatory | Italia | La Spezia | 347 m |
| Montecanal Observatory | España | Zaragoza | — |
| Monterey Accelerated Research System | — | — | — |
| Montevenere observatory | Italia | Monzuno | — |
| Monteviasco Astronomical Observatory | Italia | Curiglia con Monteviasco | — |
| Montjoia observatory | España | Moyá | — |
| Montseny Astronomical Observatory | España | Montseny | — |
| Moon Palace | Reino Unido | — | — |
 Moon Palace | Reino Unido | — | — |
| Moonbase South Observatory | Namibia | Khomas Region | 1.830 m |
| Moonedge Observatory | Estados Unidos | Northport | — |
| Moonglow observatory | Estados Unidos | Warrensburg | — |
| Mooray Observatory | Nueva Zelanda | Christchurch | — |
| Moore Observatory | Estados Unidos | Kentucky | — |
| Moorpark College Observatory | Estados Unidos | California | — |
| Moorwarfen observatory | Alemania | Jever | — |
| Moose Springs Observatory | Estados Unidos | Humbug Canal | — |
| Moquegua observatory | Perú | Distrito de Carumas | — |
| Morgantown Observatory | Estados Unidos | Morgantown | — |
| Morgan–Monroe Observatory | Estados Unidos | Indiana | — |
| Moriyama observatory | Japón | Moriyama | — |
| Morning Star observatory | Estados Unidos | Tanque Verde | — |
| Morris Observatory | Estados Unidos | Indiana | — |
| Morrison Obervatory | Estados Unidos | — | — |
| Morrison Observatory | Estados Unidos | Misuri | — |
| Mould Bay Magnetic Observatory | Canadá | — | — |
| Mounds Observatory | Estados Unidos | Oklahoma | — |
 Mount Abu Observatory | India | Distrito de Sirohi | 1.680 m |
| Mount Belleview Observatory | Estados Unidos | Canadá | — |
| Mount Burnett Observatory | Australia | Victoria | — |
 Mount Cuba Astronomical Observatory | Estados Unidos | Delaware | — |
 Mount Erebus Volcano Observatory | — | — | — |
 Mount John University Observatory | Nueva Zelanda | distrito de Mackenzie | 1.029 m |
 Mount Laguna Observatory | Estados Unidos | California | — |
 Mount Lemmon Survey | Estados Unidos | Pima County | 2.791 m |
| Mount Matajur observatory | Italia | Savogna | — |
| Mount Molehill Observatory | Nueva Zelanda | Auckland Region | — |
| Mount Nyūkasa Optical Observatory | Japón | Ina | 1.816 m |
 Mount Pleasant Radio Observatory | Australia | Tasmania | 43 m |
 Mount Stony Brook Observatory | Estados Unidos | Nueva York | — |
 Mount Suhora Observatory | Polonia | — | — |
| Mount Tarana Observatory | Australia | Bathurst | — |
 Mount Terskol Observatory | Rusia | Kabardino-Balkaria | 3.127 m |
| Mount Tuffley Observatory | Reino Unido | Gloucester | — |
| Mount Wilson-TIE observatory | Estados Unidos | monte Wilson | — |
| Mountain Creek Ranch observatory | Estados Unidos | Arizona | — |