Tornio (ˈtornio; Torneå; Duortnus ˈtuo̯rtnuːs; Tuárnus) is a city and municipality in Lapland, Finland. The city forms a cross-border twin city together with Haparanda on the Swedish side. The municipality covers an area of , of which is water. The population density is , with a total population of ().
Tornio is unilingually Finnish with a negligible number of native Swedish speakers, although this does not count vast numbers of bilinguals who speak Swedish as a second language, with an official target of universal working bilingualism for both border municipalities.
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 known in the area, similar to those found in Vuollerim (6000–5000 BC). The Swedish part of the region is not far from the oldest permanent settlement site found in Scandinavia. A former hypothesis that this region was uninhabited and colonised from the Viking Age onward has now been abandoned.
Until the 19th century, inhabitants of the surrounding countryside spoke Finnish, and Kemi Sámi, a language of the Eastern Sami group, while those of the town were mainly Swedish-speaking.
The name 'Tornio' is an old Finnish word meaning "war spear": the city is named after the river. To Swedish it was borrowed as Torneå after Torne å, an alternative name of the river.
The town received its charter from the King Gustavus Adolphus on May 12, 1621, and was officially founded on the island of Suensaari. At that time, it was the northernmost city in the world. The charter was granted 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 Sweden. 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 which came to explore the Arctic.