 Mauna Loa Observatory | Hawaii County | 3.397 m |
 Mauna Loa Solar Observatory | — | — |
| Maury Lewin Observatory | Glendora | — |
| Maynard F. Jordan Observatory | Maine | — |
| McDonald Observatory-LCO ELP | Fort Davis | — |
| McDonald Observatory-LCO ELP Aqawan A #1 | Fort Davis | — |
| McDonald Observatory-LCO ELP B | Fort Davis | — |
 McKim Observatory | Greencastle | — |
| McMillin Observatory | Ohio | — |
 Mead Observatory | Georgia | — |
| Medina Dome observatory | Medina | — |
| Mehalso Observatory | Pensilvania | — |
| Melton Memorial Observatory | Carolina del Sur | — |
| Mendel Observatory | North Andover | — |
| Mendenhall Observatory | Oklahoma | — |
| Menke Observatory | Iowa | — |
| Meyer–Womble Observatory | Colorado | — |
 Michigan State University Observatory | — | — |
 Michigan State University Observatory | Míchigan | — |
 Michigan-Dartmouth-MIT Observatory | Tucson | — |
 MicroObservatory | — | — |
| Mid Atlantic Star Party | Carolina del Norte | — |
| Middlebury College Observatory | Vermont | — |
| Middlesex School Observatory | Concord | — |
| Midway Island Magnetic Observatory | — | — |
| Mill Creek observatory | Chenoweth | — |
| Miller Observatory | Carolina del Norte | — |
 Millimeter Anisotropy eXperiment IMaging Array | — | — |
 Millstone Hill Observatory | Massachusetts | — |
| Millwood Observatory | Center Point (Texas) | — |
 Milwaukee Astronomical Society Observatory | New Berlin | — |
 MINERνA | — | — |
| MiniBooNE | — | — |
| Minnetonka observatory | Minnetonka | — |
| MINOS+ | — | — |
| Missouri City observatory | Missouri City | — |
| Moka Observatory | Benson | — |
| Moonedge Observatory | Northport | — |
| Moonglow observatory | Warrensburg | — |
| Moore Observatory | Kentucky | — |
| Moorpark College Observatory | California | — |
| Moose Springs Observatory | Humbug Canal | — |
| Morgantown Observatory | Morgantown | — |
| Morgan–Monroe Observatory | Indiana | — |
| Morning Star observatory | Tanque Verde | — |
| Morris Observatory | Indiana | — |
| Morrison Obervatory | — | — |
| Morrison Observatory | Misuri | — |