| 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 | — |
| 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 | — |
| 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 | — | — | — |
| 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 | — |
 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 | — |
| Mountain Skies Observatory | Estados Unidos | Wyoming | — |
| Mountainville Observatory | Estados Unidos | Utah | — |
| MPO Observatory | Estados Unidos | Florissant | — |
| Mr. Barclay’s observatory | Reino Unido | Essex | — |
 Mt Etna Volcano Observatory | Italia | — | 2.940 m |
| Mt. Bigelow Station 27" Telescope | Estados Unidos | Pima County | — |
| Mt. Guizi observatory | República Popular China | Hongshan | — |
| Mt. Kajigamori observatory | Japón | Ōtoyo | — |
| Mt. Ohdu Unification Observatory | Corea del Sur | Paju | — |
| Mueggelheim observatory | Alemania | Parada de tranvía Müggelheimer Straße/Wendenschloßstraße, Berlin | — |
| Muensterschwarzach Observatory | Alemania | Schwarzach am Main | — |
| Mui Observatory | Japón | — | — |
 Mukai Chiai Children's Science Museum | Japón | Tatebayashi | — |
 Mullard Radio Astronomy Observatory | Reino Unido | Harlton | — |
| Multa Observatory | Rusia | Multa | — |
| Mundenheim observatory (1907-1913) | Alemania | Ludwigshafen | — |
| Munich observatory (1819-1938) | Alemania | Múnich | — |
| Murcia Observatory | España | Murcia | — |
| Murillo Family Observatory | Estados Unidos | San Bernardino | — |
| Murou observatory | Japón | Emperador Uda | — |
| Murray Hill Observatory | Estados Unidos | Beaverton | — |
| Murrumbateman observatory | Australia | Murrumbateman | — |
| Murviel-les-Montpellier observatory | Francia | Murviel-lès-Montpellier | — |
 Museo del Espacio de Hong Kong | República Popular China | Hong Kong | — |
 Museo La Specola | Italia | Padua | — |
 Muses B | — | — | — |
| Muxagata Observatory | Portugal | Vila Nova de Foz Côa | — |
 Mykolaiv Observatory | Ucrania | Mykolaiv | 52 m |
 Mårtsbo observatory | Suecia | Municipio de Gävle | — |
 Mühlenturm | Alemania | Rees | — |