Kajaani (Kajana in Swedish) is a town in Finland.

It is located in the province of Oulu and is the capital of the Kainuu region. It is located southeast of Oulujärvi (Lake Oulu), which drains to the Gulf of Bothnia along Oulujoki (Oulu river). The city has a population of 35 964 and covers an area of 1369 km² of which 212 km² is water. The population density is 31 inhabitants per km². The city is unilingually Finnish.

Kajaani is in the center of the region of Kainuu. The folklorist Elias Lönnrot began his travels from here when collecting folklore in Karelia.

Kajaani was founded in 1651, as one of the Finnish cities founded by the Governor General of Finland Per Brahe. At that time, the Kainuu region — as wood country — was an important wood tar producer, and the tar trade was the major industry. The barrels of tar were shipped to Oulujärvi and then along the Oulujoki to Oulu in the Bothnian coast of the Baltic sea. From there, the tar was shipped to continental Europe, where it was used as a sealant and an anti-rot agent for ship hulls.

The development of population numbers (December 31):

• 1987 - 36,056
• 1990 - 36,428
• 1997 - 36,541
• 2000 - 36,088
• 2002 - 35,842

Important industries in the city today are the wood industry and the paper industry . An important employer in the city is also the Kainuu Brigade , is the largest garrison in Finland — the Brigade employs some 600 people and trains 4000 conscripts a year. There is a polytechnic institute in the city.

One famous historical monument in Kajaani is the ruins of the Kajaani castle , located on the Kajaani river in the centre of the city. The castle was originally built in 1604 and completed in 1619. The castle served as an administrative centre, prison, military base and refuge for the citizens.

During the Greater Wrath in the 18th century, the castle was forced to surrender to Russian forces. The Russians exploded the castle in March 1716 and it has been in ruins ever since. Today, the ruins are especially popular among children, who like to explore the dark, twisty little passages and have fantasy adventures.

A sister city of Kajaani is Marquette, Michigan.