| L'Ampolla observatory | España | La Ampolla | — |
| L'Aquila Magnetic Observatory | Italia | — | 682 m |
| La Avejerilla observatory | España | Las Palmas | — |
| La Axarquia Observatory | España | Chilches | — |
 La capitale Observatory | Canadá | Quebec | — |
| La Chaux de Fonds observatory | Suiza | La Chaux-de-Fonds | — |
| La Corte Observatory | España | La Antigua | — |
| La Couyere astronomical center | Francia | La Couyère | — |
| La Fecha Observatory | España | Arroyo de la Encomienda | — |
| La Giustiniana observatory | Italia | Roma | — |
| La Mata Observatory | España | La Mata | — |
| La Palma-NEON Observatory | España | Río Gallegos | — |
 La Posta Astro-Geophysical Observatory | Estados Unidos | Condado de San Diego | — |
| La Puebla de Vallbona observatory | España | España | — |
| La Romaneta observatory | España | — | — |
| La Sagra Observatory | España | Don Domingo | — |
| La Senda observatory | España | Cabanillas del Campo | — |
| La Seyne sur Mer observatory | Francia | La Seyne-sur-Mer | — |
| La Vara observatory | España | Muñás | — |
 Laboratoire atmosphères, milieux, observations spatiales | Francia | Guyancourt | — |
 Laboratorio Lincoln | Estados Unidos | — | — |
| Laboratory of Planetology and Geosciences | Francia | Nantes | — |
| LaCaille observatory | Sudáfrica | centurión | — |
 Ladd Observatory | Estados Unidos | Providence | — |
| Lahti Observatory | Finlandia | Lahti | — |
| Laietania observatory | España | Parets del Vallès | — |
| Lake Afton Public Observatory | Estados Unidos | Kansas | — |
| Lake Arrowhead observatory | Estados Unidos | Lake Arrowhead | — |
| Lake Clear observatory | Estados Unidos | Nueva York | — |
| Lake Forest observatory | Estados Unidos | El Dorado Hills | — |
| Lake of the Woods Observatory | Estados Unidos | Lake of the Woods | — |
| Lake Saint Louis observatory | Estados Unidos | Lake St. Louis | — |
| Lakeland Astronomical Observatory | Estados Unidos | Kirtland | — |
| Lamalou-les-Bains observatory | Francia | Lamalou-les-Bains | — |
 Lamont–Doherty Earth Observatory | Estados Unidos | Palisades | — |
| Lamont–Hussey Observatory | Sudáfrica | — | — |
| LAMP observatory | Estados Unidos | New River | — |
| Lancaster observatory | Reino Unido | Lancaster | — |
 Land's End Observatory | Estados Unidos | Colorado | — |
| Landehen observatory | Francia | Landéhen | — |
| Landolt Astronomical Observatory | Estados Unidos | Luisiana | — |
| Langkawi Look Out Point | Malasia | Langkawi District | — |
| Langkawi National Observatory | Malasia | Kedah | — |
| Languard Bird Observatory | Reino Unido | — | — |
| Lanzhou Magnetic Observatory | República Popular China | — | — |
| Large Fiber Array Spectroscopic Telescope | Estados Unidos | Arizona | — |
| Large High Altitude Air Shower Observatory | República Popular China | — | — |
| Large Volume Detector | Italia | — | — |
| Las Brisas Observatory | Estados Unidos | Colorado | — |
| Las Negras observatory | España | Las Negras | — |
| Las Vaguadas observatory | España | Las Vaguadas | — |
| Laupheim Planetarium | Alemania | Laupheim | — |
| Laws Observatory | Estados Unidos | — | — |
 Lazkuako behatokia | España | — | — |
| Lazuli Space Observatory | — | — | — |
| Le Couvent de Lentin observatory | Francia | Lédergues | — |
| Le Cres observatory | Francia | Le Crès | — |
| Le Creusot observatory | Francia | Le Creusot | — |
| Leamington Spa observatory | Reino Unido | Royal Leamington Spa | — |
 Lear Jet Observatory | — | — | — |
| Learmonth Magnetic Observatory | Australia | Western Australia | — |
| Learmonth Solar Observatory | Australia | Western Australia | — |
| Leavenworth observatory | Estados Unidos | Leavenworth | — |
| Lee Observatory | Líbano | Beirut | 35 m |
| Leeside Observatory | Canadá | Lower Rideau Lake | — |
| Leiden Southern Station | Sudáfrica | Johannesburgo | 1.234 m |
| Leipzig observatory (since 1861) | Alemania | Leipzig | — |
| Lelekovice observatory | República Checa | Lelekovice | — |
| Lenkerbeck observatory | Alemania | Essen | — |
| Lenomiya observatory | Estados Unidos | Casa Grande | — |
| Leo Observatory | Estados Unidos | Tortolita | — |
 Leopold-Figl-Observatorium | Austria | Altenmarkt an der Triesting | — |
| Lerwick Magnetic Observatory | Reino Unido | Islas Shetland | — |
| Les Barres Observatory | Francia | Lamanon | — |
| Les Gavarres Astronomical Observatory | España | San Martivell | — |
| Les Pedritxes observatory | España | Matadepera | — |
| Les Planes de Son observatory | España | Alto Aneu | — |
| Les Tardieux observatory | Francia | Saint-Pierre-Lafeuille | — |
| Leura observatory | Australia | Leura | — |
| Leuschner Observatory | Estados Unidos | Lafayette | — |
| Levendaal observatory | Países Bajos | Leiden | — |
 Leviatán de Parsonstown | Irlanda | Townparks | — |
| Leyland Observatory | Reino Unido | South Ribble | — |
| Libbiano astronomical observatory | Italia | Libbiano | — |
| Light Climatic Observatory | Suiza | Arosa | — |
| LightBuckets observatory | Australia | Pingelly | — |
| LightBuckets Observatory | Estados Unidos | Rodeo | — |
 LIGO Hanford Observatory | Estados Unidos | Hanford Site | — |
| LIGO Livingston Observatory | Estados Unidos | Livingston | — |
| Lijiang Station | República Popular China | Gucheng | — |
 Lilienthal Observatory | Alemania | Lilienthal | — |
 Lille observatory of astronomy | Francia | Lille | — |
| Lime Creek Observatory | Estados Unidos | James A. Garfield | — |
| Linceo observatory | España | Santander | — |
| Lindby observatory | Suecia | Torsjö | — |
| Linden Observatory Complex | Australia | New South Wales | — |
 Lindenberg Meteorological Observatory | Alemania | Brandemburgo | — |
 Lindheimer Astrophysical Research Center | Estados Unidos | Evanston | — |
| Linhaceira observatory | Portugal | Praia do Ribatejo | — |
| Lionel José Andriatto Didactic Astronomy Observatory | Brasil | Bauru | — |
| LiShan Observatory | República Popular China | Lintong | — |
| LiteBIRD | Japón | — | — |
| Little Moose Observatory | — | — | — |
| Live Oak Observatory | Estados Unidos | Pontotoc | — |
| Livergnano observatory | Italia | Pianoro | — |
| Liverpool observatory (before 1867) | Reino Unido | Liverpool | — |
| Liverpool observatory (since 1867) | Reino Unido | Liverpool | — |
| Livorno observatory | Italia | Livorno | — |
| Lledoner observatory | España | ordalía | — |
| Llica d'Amunt Observatory | España | Llissá de Munt | — |
| Lo Fossil Observatory | España | Ager | — |
 LOFAR | Países Bajos | — | — |
| Lofoten-Vesterålen Ocean Observatory | Noruega | — | — |
| LOFT | — | — | — |
| Lohbach Observatory | Alemania | Dortmund | — |
 Lohrmann Observatorium | Alemania | Dresde | 140 m |
| Lohrmann observatory | Alemania | Dresde | — |
| Loiano observatory | Italia | Loiano | — |
| Lomazzo Observatory | Italia | Bissago | — |
 Lomnický Štít Observatory | Eslovaquia | Lomnický štít | 2.632 m |
| Lone Star observatory | Estados Unidos | Bentley | — |
| Long Baseline Observatory | Estados Unidos | — | — |
| Long Valley Observatory | Estados Unidos | — | — |
| Longa Vista observatory | Brasil | Bragança Paulista | — |
| Lonjsko Polje Magnetic Observatory | Croacia | — | — |
| Loomberah observatory | Australia | Loomberah | — |
 Loomis Observatory | Estados Unidos | — | — |
| Loose Goose Observatory | Canadá | Saint-Jérôme | — |
| Los Altos de Arguineguin Observatory | España | Mogán | — |
| Los Cabezones observatory | Argentina | Santa Rosa | — |
| Los Campitos observatory | Argentina | Cañuelas | — |
| Los Milanos observatory | España | Cáceres | — |
| Loudwater observatory | Reino Unido | Loudwater | — |
 Louis-Théophile Moreux observatory | Francia | Bourges | — |
| Lovoe Magnetic Observatory | Suecia | Lovön | — |
| Lozova school astronomical observatory | Ucrania | Lozova | — |
 Lubomir Astronomical Observatory | Polonia | — | — |
| Lucca observatory | Italia | Lucca | — |
| Luiz Cruls Astronomical Observatory | Brasil | Park Way | — |
| Lulin Widefield Telescope | República de China | Sinyi Township | — |
 Lumezzane observatory | Italia | Lumezzane | — |
| Lumijoki observatory | Finlandia | Lumijoki | — |
| Lunar Cafe Observatory | Estados Unidos | Míchigan | — |
| Lunar Infrastructure for Exploration | — | — | — |
| Lunping Magnetic Observatory | República de China | — | — |
| Luscherz Observatory | Suiza | Lüscherz | — |
| Lviv Magnetic Observatory | Ucrania | — | — |
 Lviv Polytechnic Observatory | Ucrania | Leópolis | — |
| Lvye Observatory | República Popular China | Huqiu | — |
| Lycksele Magnetic Observatory | Suecia | — | — |
 Lynx X-ray Surveyor | — | — | — |
| Lågskär bird observatory | Finlandia | Lemland | — |
 Lübeck observatory | Alemania | Lübeck | 10 m |