Lahti is a city and municipality in Finland. It is the capital of the region of Päijänne Tavastia and its growing region is one of the main economic hubs of Finland. Lahti is situated on a bay at the southern end of lake Vesijärvi about 100 kilometres (60 mi) north-east of the capital city Helsinki and 74 kilometres (46 mi) east of Hämeenlinna, the capital of the region of Tavastia Proper. It is also situated at the intersection of Highway 4 (between Helsinki and Jyväskylä) and Highway 12 (between Tampere and Kouvola), which are the most significant main roads of Lahti.

In English, the Finnish word Lahti literally means bay. Lahti is also dubbed the "Chicago of Finland" due to the premise of both cities, when they were known as "slaughterhouse cities". Also, the troubled history of both cities in the field of crime has been seen as one of the similarities.

Lahti is a long-time pioneering city in environmental sustainability, dating back to as early as 1990 and before. The European Commission has named Lahti as the European Green Capital of 2021.

The coat of arms of the city depicts a train wheel surrounded by flames.

The terrain of Lahti is dominated by the first Salpausselkä ridge that cuts through the city from west to east. The city is located in the transition from the southern coastal area to the Finnish Lakeland; prominent to the north of the Salpausselkä are rocky hills and fragmented lakes, while its south side is dominated by forests and small rivers. The divide is also apparent in the soil, which mostly consists of till in the north and clay in the south. The biggest lake is Vesijärvi which also is a gateway to the Central Finland via Lake Päijänne. There is also pond called Pikku-Vesijärvi near the Lanu-puisto park.