The delta of the Torne river has been inhabited since the end of the last ice age, and there are currently (1995) 16 settlement sites (boplatsvallar) known in the area, similar to those found in Vuollerim (c.6000–5000 BC). The Swedish part of the region is not far from the oldest permanent settlement site found in Scandinavia. A former theory that this region was uninhabited and "colonized" from the Viking Age onward has now been abandoned.
The church spire at Tornio was one of the landmarks used by de Maupertuis in his measurements. The church was constructed in 1686 by Matti Joosepinpoika Härmä.
Until the 19th century, inhabitants of the surrounding countryside spoke Kemi Sami, a language of the Eastern Sami group similar to Finnish, while those of the town were mainly Swedish-speaking.
Tornio was named Torneå in Swedish after Torne å, an alternative name of the river, and later fennicized Tornio.
The town received its charter from the King of Sweden in 1621 and was officially founded on the island of Suensaari (literally "Wolf Island", probably named after one of the main landowners of the past). The charter was in recognition of Tornio being the hub of all trade in Lapland throughout the 16th century. It was the largest merchant town in the North at the time and for some years ranked as the richest town in the Swedish realm. Despite the lively trade with Lapland and overseas, the population of the town remained stable for hundreds of years at little over 500.
During the 18th century Tornio was visited by several expeditions from Central Europe who came to discover the Arctic. The most notable expedition (1736–1737) was led by a member of the Académie française, Pierre Louis Moreau de Maupertuis, who came to take meridian arc measurements along the Torne River Valley which would show that the globe is flattened towards the poles.
The Lapland trade on which Tornio depended started to decline in the 18th century,