The area of what is now Thun was inhabited since the Neolithic age (mid-3rd millennium BC). The name of the city derives from the Celtic term Dunum, meaning "fortified city". It fell to Rome in 58 BC, when Roman legions conquered almost all of Switzerland, and it soon became one of the main centres of Roman administration in the region.
The Romans were driven out of Thun, and out of the rest of Switzerland, by the Burgundians around 400 AD. The Aar became the frontier between the Christian Burgundians and the Pagan, German-speaking Alemanni, who lived north. Thun was mentioned for the first time during the 7th century, in the chronicle of Frankish monkFredgar.
The region of Thun became a part of the Holy Roman Empire in 1033, when Conrad IIgained the title of King of Burgundy. The emperors entrusted the Z�hringen family, centred in Bern, with subduing the unruly nobles of central Switzerland. Around 1190 Duke Bertold V of Z�hringen, built a castle in Thun and expanded the city. After Bertold's death in 1218, his territories went to Ulrich III von Kyburg.
In 1264 Thun received city rights and in 1384 the town was bought by the canton of Bern. Thun was the capital of the Canton of Oberland of the Helvetic Republic, which lasted from 1798 until 1803.
In 1819 a Military School was founded in the city, which later developed into the main military school in Switzerland. Thun was connected to the railway network of Switzerland in 1859 and telephone access made available in 1888
