| 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 | — |