M
| Name | Established | Location |
| Macalester College Observatory | Saint Paul, Minnesota, USA | |
| Magdalena Ridge Observatory (under construction) | 2006 | Socorro County, New Mexico, USA |
| Malakoff Tower | 1845 | Recife, Brazil |
| Majorca Observatory | 1991 | Majorca, Spain |
| Manastash Ridge Observatory | 1972 | Ellensburg, Washington, USA |
| Maragheh Observatory | 1259 | Maragheh, Iran |
| Margaret M. Jacoby Observatory | 1978 | Warwick, Rhode Island, USA |
| Maria Mitchell Observatory | 1908 | Nantucket, Massachusetts, USA |
| Markree Observatory | 1837 | County Sligo, Ireland |
| Marseille Observatory | Marseille, France | |
| Martz Observatory | Chautauqua County, New York, USA | |
| Mauna Kea Observatory | 1967 | Mauna Kea, Hawaii, USA |
| Mauna Loa Solar Observatory | Hawaii, USA | |
| Maynard F. Jordan Observatory | 1901 | Orono, Maine, USA |
| McDonald Observatory | Davis Mountains, Texas, USA | |
| McKim Observatory | 1884 | Greencastle, Indiana, USA |
| McMath–Hulbert Solar Observatory | 1930 | Lake Angelus, Michigan, USA |
| MDM Observatory | Kitt Peak, Arizona, USA | |
| Mead Observatory | 2001 | Columbus, Georgia, USA |
| Mees Observatory | Rochester, New York, USA | |
| Mehalso Observatory | Erie, Pennsylvania, USA | |
| Melbourne Observatory | 1862 | Melbourne, Victoria, Australia |
| Melton Memorial Observatory | 1928 | Columbia, South Carolina, USA |
| Mendenhall Observatory | 2002 | Stillwater, Oklahoma, USA |
| Menke Observatory | 1994 | Davenport, Iowa, USA |
| Metsähovi Observatory | 1966 | Kirkkonummi, Finland |
| Meudon Observatory | 1876 | Meudon, France |
| Meyer–Womble Observatory | 1996 | Mount Evans, Colorado, USA |
| Michigan State University Observatory | 1969 | East Lansing, Michigan, USA |
Observatoire Midi-Pyrénées
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Headquarters: Toulouse, France
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| Miller Observatory | 1976 | Maiden, North Carolina, USA |
| Mills Observatory | 1935 | Balgay Hills, Dundee, Scotland |
| MMT Observatory | 1987 | Mount Hopkins, Arizona, USA |
| Modra Observatory | 1988 | Modra, Slovakia |
| Moletai Astronomical Observatory | 1969 | Moletai, Lithuania |
| Molonglo Observatory Synthesis Telescope (MOST) | 1960 | Bungendore, New South Wales, Australia |
| Mont Mégantic Observatory | 1978 | Mont Mégantic, Québec, Canada |
| Monterey Institute for Research in Astronomy | 1972 | Monterey, California, USA |
| Moore Observatory | 1978 | Oldham County, Kentucky, USA |
| Morgan–Monroe Observatory | 1966 | Morgan–Monroe State Forest, Indiana, USA |
| Morris Observatory | 1973 | North Manchester, Indiana, USA |
| Morrison Observatory | Fayette, Missouri, USA | |
| Moscow Cosmic-Ray Station | Troitsk, Russia | |
| Mount Stony Brook Observatory | 1968 | Stony Brook, New York, USA |
| Mounds Observatory | Tulsa, Oklahoma, USA | |
| Mount Abu Observatory | Mount Abu, Rajasthan, India | |
| Mount Cuba Astronomical Observatory | Greenville, Delaware, USA | |
| Mount Graham International Observatory | 1993 | Mount Graham, Arizona, USA |
| Mount John University Observatory | 1965 | Lake Tekapo, South Island, New Zealand |
| Mount Laguna Observatory | 1968 | San Diego, California, USA |
| Mount Lemmon Observatory | Tucson, Arizona, USA | |
| Mount Pleasant Radio Observatory | Tasmania | |
| Mount Stromlo Observatory | 1911 | Canberra, Australia |
| Mount Suhora Observatory | 1987 | Poland |
| Mount Wilson Observatory | 1904 | Mount Wilson, Los Angeles County, California, USA |
| Mountain Skies Observatory | 1997 | Lyman, Wyoming, USA |
| Mullard Radio Astronomy Observatory | 1957 | Cambridge, England |
Read more about this topic: List Of Astronomical Observatories