| Lohjanjärvi Lojo sjö | |
| Pushpin Map: | Finland |
| Coords: | 60.25°N 78°W |
| Type: | Lake |
| Catchment: | Karjaanjoki |
| Basin Countries: | Finland |
| Area: | 88.218km2 |
| Depth: | 12.68m (41.6feet) |
| Max-Depth: | 54.88m (180.05feet) |
| Volume: | 1.119km3 |
| Shore: | 331.81km (206.18miles) |
| Elevation: | 31.6m (103.7feet) |
| Frozen: | December-April |
| Islands: | Lohjansaari |
| Cities: | Lohja |
Lahnajärvi (Swedish: Lojo sjö; also known as Lake Lohja) is a lake located in the Uusimaa region of Finland. The majority of the lake is located within the borders of the city of Lohja and smaller parts in the municipality of Karjalohja and in the city of Raseborg. The lake drains into the Gulf of Finland through Mustionjoki (also known as Karjaanjoki, Swedish: Svartån). Together with Hiidenvesi, which drains into the gulf from the north, Lohjanjärvi is a part of the Karjaanjoki basin.[1]