Hiisi (Finnish pronunciation: [ˈhiːsi]; plural hiidet [ˈhiːdet]) is a term in Finnic mythologies, originally denoting sacred localities and later on various types of mythological entities.

In later, Christian-influenced folklore, they are depicted as demonic or trickster-like entities, often the autochthonous, pagan inhabitants of the land, similar in this respect to mythological giants. They are found near salient promontories, ominous crevasses, large boulders, potholes, woods, hills, and other outstanding geographical features or rough terrain.

Hiisi was originally a spirit of hill forests (Abercromby 1898). In Estonian hiis (or his) means a sacred grove in trees, usually on elevated ground. In the spells ("magic songs") of the Finns the term Hiisi is often used in association with a hill or mountain, as a personage he also associated with the hills and mountains, such as the owner or ruler of the same. His name is also commonly associated with forests, and some forest animals.

More recent speculation, by Mauno Koski and others, associates the Finnish "Hiisi" and the Estonian "Hiis" primarily with burial sites, or sacred areas associated with burial sites; with a secondary meaning of hiisi applied to dominant, exceptional or anomalous geographical features. Following the Christianization of Finland and Estonia these meanings may have been lost or become unclear, which may have led to the application of anthropomorphism to hiisi sites, as "giant's ...", or the emergence of Hiisi or Hiis as a proper noun, the name of a deity or spirit.

It has been supposed that Hiisi's evil nature has been magnified over time, starting with the Christianization of Finland in the 12th and 13th centuries. In more recent times his nature has been nearly identical with that of a Christian devil, or even the Devil. In Bishop Mikael Agricola's list of Finnish pagan gods Hiisi is said to have been a god of forest game or fur, sharing this attribute with a similar god, Tapio.

Oral folklore concerning hiisi mostly describes creatures that dwelt in hiisi sites, typically trolls or giants. Many of the stories describe how odd rock formations or other features of the landscape were created by the actions of these creatures.

Much of the recorded folklore about hiisi comes from the village of Narva in Vesilahti, Finland. There are tales of cauldrons of coins being caught by fishermen after having rolled down the cliff at Hiidennokka, as well as tales of how the cape of Hiidennokka was created by giants throwing rocks. One of these tales takes on a Christian element, as the giants throw rocks into the sea to prevent people going to church by boat.

Later the original aspect of nature's awesomeness inherent in the hiidet was diminished, and they passed into folklore as purely evil spirits vaguely analogous to trolls. According to this later view, hiidet were often small in size, but others were gigantic. Hiidet could travel in a noisy procession and attack people who did not give way to them. If somebody left their 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 that were blessed in contrast to the pagan holiness residing in the awesome and forbidding features of raw nature, and an evil hiisi could not step inside areas sanctified by cultivation.^{[citation needed]}