In the Middle Ages, Jämsä was the most northern administrative parish of Tavastia province (Häme, in Finnish). Back then, Jämsä was much larger than nowadays, because it included the settlements of Jämsä, Jämsänkoski, Koskenpää, most of current Korpilahti and Muurame, and parts from Säynätsalo and Petäjävesi. Historically, this territory is known as Great-Jämsä (Suur-Jämsä). The centre of Great-Jämsä settlement was located around the current church of Jämsä. The first municipal meeting was held at Jämsä in 1866, and therefore this year is considered to be the beginning of the independent municipal administration of Jämsä.
In 1860’s many areas of the former parish separated as independent municipalities. Eventually, Great-Jämsä was decreased to a territory consisting only Jämsä, Jämsänkoski, and Koskenpää. This period of so called “Old Jämsä” lasted until 1926.
The municipality of ”Old Jämsä” fell apart in the beginning of 1926, when Koskenpää and Jämsänkoski left the parish. Jämsä continued alone as a separate, independent administration up to the year 2001. In 1977 the status of Jämsä was raised from a market-town to a city.
In the beginning of the 21st century, new municipal changes expanded the territory of Jämsä. In the beginning of 2001, the City of Jämsä and the Municipality of Kuorevesi formed a new City of Jämsä. In the beginning of 2007, parts of the Municipality of Längelmäki were joined with Jämsä, and in the beginning of the year 2009, the areas of so called “Old Jämsä” returned back home, when the City of Jämsä and the City of Jämsänkoski formed one and united city. The name of the city remained as Jämsä.
More information about the history of Jämsä can be found on the web pages of local cultural heritage: Museum24