Hiisi are a kind of tutelary spirits in mythologies of the Baltic Sea area, especially in Finland. Originally, the term meant "holy place". In the related Estonian language 'Hiis' still means sacred forest. Often, the English "goblin" is translated as hiisi in Finnish, due to the numerous similarities between the typical goblin and hiisi. Hiisi has also been used as a translation of "orc" in Tolkien's books.

The chief Hiisi is helped by a number of lesser demons called hiiet in the Kalevala. Most often they are considered to be malicious or at least very horrifying. Later the original aspect of nature's awesomeness inherent in the hiisi was diminished, and they passed into folklore as purely evil spirits. According to this later view, Hiisi were often small in size, on some occasions gigantic.

They are found near salient promontories, ominous crevasses, large boulders, potholes, woods, hills, and other awesome geographical features or rough terrain.

Hiisi could travel in a noisy procession, and attack people who did not give way to them. If somebody left his door open, a Hiisi could come inside and steal something. If you were chased by a Hiisi you should seek safety in a cultivated area. In folklore, it was the cultivated areas which were blessed in contrast to the pagan holiness residing in the awesome and forbidding features of raw nature, and evil hiisi could not step inside areas sanctified by human cultivation.

Pre-historic stone structures and large stone boulders were thought to have been erected by Hiisi or giants. The Finnish term for an Iron Age grave (consisting of a pile of rocks) is still called a hiidenkiuas, Hiisi's pile of rocks.