 Kirkwood Observatory | Indiana | — |
| Kitt Peak-Bok observatory | Nelson | — |
 Kopernik Space Center | Vestal | 530 m |
 Kuiper Airborne Observatory | — | — |
| Kuiper Telescope | Pima County | 2.510 m |
 La Posta Astro-Geophysical Observatory | Condado de San Diego | — |
 Laboratorio Lincoln | — | — |
 Ladd Observatory | Providence | — |
| Lake Afton Public Observatory | Kansas | — |
| Lake Arrowhead observatory | Lake Arrowhead | — |
| Lake Clear observatory | Nueva York | — |
| Lake Forest observatory | El Dorado Hills | — |
| Lake of the Woods Observatory | Lake of the Woods | — |
| Lake Saint Louis observatory | Lake St. Louis | — |
| Lakeland Astronomical Observatory | Kirtland | — |
 Lamont–Doherty Earth Observatory | Palisades | — |
| LAMP observatory | New River | — |
 Land's End Observatory | Colorado | — |
| Landolt Astronomical Observatory | Luisiana | — |
| Large Fiber Array Spectroscopic Telescope | Arizona | — |
| Las Brisas Observatory | Colorado | — |
| Laws Observatory | — | — |
| Leavenworth observatory | Leavenworth | — |
| Lenomiya observatory | Casa Grande | — |
| Leo Observatory | Tortolita | — |
| Leuschner Observatory | Lafayette | — |
| LightBuckets Observatory | Rodeo | — |
 LIGO Hanford Observatory | Hanford Site | — |
| LIGO Livingston Observatory | Livingston | — |
| Lime Creek Observatory | James A. Garfield | — |
 Lindheimer Astrophysical Research Center | Evanston | — |
| Live Oak Observatory | Pontotoc | — |
| Lone Star observatory | Bentley | — |
| Long Baseline Observatory | — | — |
| Long Valley Observatory | — | — |
 Loomis Observatory | — | — |
| Lunar Cafe Observatory | Míchigan | — |
 Macalester College Observatory | Saint Paul | — |
| Madison-YRS observatory | Wisconsin | — |
| Maiden Lane observatory | Bainbridge Island | — |
| Manastash Ridge Observatory | Washington | — |
| Manastash Ridge Radar | Washington | — |
| Margaret M. Jacoby Observatory | Rhode Island | — |
| Mark Evans Observatory | Bloomington | — |
| Mark Slade Remote Observatory | Virginia | — |
| Martin Observatory | Blacksburg | — |
| Martz Observatory | Nueva York | — |
| Maryland Space Grant Consortium Observatory | Baltimore | — |