| 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 | — |