Lohja is a city and municipality in the Uusimaa region of Finland. The city has a population of 47,518 (2017), and it covers an area of 1,109.73 square kilometres (428.47 sq mi) of which 91.78 km² (35.44 sq mi), or 8.3 percent, is water. The population density of Lohja is 131.34 inhabitants per square kilometre (340.2/sq mi). The municipality is bilingual, with the majority being Finnish and minority Swedish speakers. Lohja has the fourth-most summer houses of any municipality in Finland, with 8,468 located within the city as of June 2018. Lohja is located near Greater Helsinki, and it benefits from a good road network. It takes less than an hour to drive from Helsinki to Lohja on the E18 motorway, which is one of the most significant main road connections in Lohja next to Hangonväylä.

The landscape of Lohja is characterized by manors and gardens. Its area is divided by the Lohja ridge, which forms a watershed for the largest lake system in Uusimaa, Lohjanjärvi; mostly that's why Lohja is also referred to as "Lake City" (järvikaupunki). The medieval Church of St. Lawrence is the architectural highlight of downtown Lohja, which also includes a heterogeneous mix of buildings mostly dating from the 1960s onwards. The Lohja library, which was opened in 2005, is a distinctly modern building placed in the very centre of the city.

Lohja has been a focal point for the population and economy of western Uusimaa since the early 14th century. It was renowned as a trading centre in the Middle Ages.[citation needed] The local inhabitants were among the pioneers of the Finnish mining and construction material industries. Lohja has long-established traditions in horticulture and especially in market gardening. These traditions are represented by the symbols of present-day Lohja: limestone and an apple.

During the Second World War, the Soviet Union heavily bombed the city, attempting to destroy the Tytyri limestone mine.