Posio is a municipality in the southern part of the province of Lapland. The municipality has a population of 3,167 and covers an area of ​​3,544.90 km², of which 505.11 km² are water bodies. The population density is 1.04 inhabitants / km².

The word Posio, which is the name of the municipality, means an opening in the back of a Sámi castle, which also has a mythical meaning. Posio is also used as a house, village and lake name and surname.

The share of water bodies (14.2%) is the largest among the municipalities in the province of Lapland. The water bodies have attracted a lot of leisure time to the municipality.

Posio's neighboring municipalities are Rovaniemi, Kemijärvi, Ranua, Salla, Kuusamo, Taivalkoski and Pudasjärvi. The distance to the center of Posio is 220 kilometers from Oulu, 132 kilometers from Rovaniemi and 59 kilometers from Kuusamo, where the nearest airport is located. The main road 81 and highway 5 (E63) pass through Posio.

Posio's most important industry is services. Agriculture has remained significant. Forest and fish resources are large. In 2015, 64.3% of the employed labor force living in the municipality worked in services, 22.4% in primary production and 11.4% in processing.

On the map of Finnish dialects, Posio is the only municipality in the province of Lapland to belong to the Kainuu dialects of Savo dialects for reasons of settlement history.