Ilmarinen (pronounced [ˈilmɑrinen]) is a god and archetypal artificer from Finnish mythology. He is immortal and capable of creating practically anything. In addition, he is described as working the known metals of the time, including brass, copper, iron, gold, and silver. The great works of Ilmarinen include the crafting of the dome of the sky and the forging of the Sampo.

Ilmarinen is the successor of the original sky god from West Uralic mythology, as well as the personification of the sky. In runic songs, many of his roles were later shifted to the new sky god Ukko, but both became less important than the new Odin-resembling hero Väinämöinen. While Ukko gained the role of a controller of rain and weather for the purposes of agriculture, Ilmarinen continued to be regarded as a wind god.

In the Kalevala, he is a blacksmith and an inventor. His usual epithet in the Kalevala is seppä or seppo ("smith"), which is the source of the given name Seppo.

Cognates of the Finnish word ilma ('air') are attested in almost all the main Finno-Ugric languages apart from the Mari and Mordvinic languages, allowing the reconstruction of proto-Finno-Ugric *ilma meaning something like 'sky'. This noun is also attested as the name of a god in Khanty (Num-Iləm), Komi (Jen), Udmurt (Inmar) and the Finnic languages, suggesting that proto-Finno-Ugric likewise had a sky god credited with creating the sky called *Ilma. In Proto-Finnic, the suffix -r(i), which is used to form words for people associated with the root word, was added to *ilma to give the god-name *Ilmar(i) ('Sky-being'); rare attestations of similar forms such as Udmurt Ilmar and Sámi Ilmaris seem to be loans from Finnic. The name could've also existed among Sámi as a thunder god Ilmaričče. In Kalevala metre poetry, the diminutive suffix -nen enabled the formation of the name Ilmarinen, which neatly fills two trochaic feet and so became the dominant form of the name in that tradition.

According to Eugene Helimski and Vladimir Napolskikh, Proto-Uralic religion had a concept of a creator sky god who did not effect things much after the creation of the world, only providing stability and continuity somewhere in the background, almost like a deus otiosus. He was usually not approached directly, and it was more common that deities of nature and the underworld were asked to help instead. As the forger of the sky, Ilmarinen fits as a successor of this Proto-Uralic sky god. He is believed to have taken on the qualities of a smith through the Proto-Finnic contact with iron-working cultures, such as the Indo-European Balts or speakers of Common Germanic.

Ilmarinen is also directly appealed to for aid in several incantation runes. Insofar as Elias Lönnrot heavily redacted the original runes collected by him and others, it's valuable to differentiate between the Kalevala and the original poems sung by rune singers [fi].

The first time Ilmarinen was mentioned in writing was by Mikael Agricola in 1551, who called Ilmarinen as the one who "brought peace and air, and took travellers forward" (Ilmarinen Rauhan ja ilman tei / ja Matkamiehet edheswei.). In 1663, Johan Cajanus wrote that in folk belief, Ilmarinen was one of the 12 sons of the giant Kaleva. Despite Agricola's description, Ilmarinen rarely has the role as a helper of sailors in runic songs; only one such runic song has been recorded (from North Ostrobothnia and Kainuu) but its structure has raised doubts about if it was truly initially formed as oral tradition among the common folk. According to Kaarle Krohn, being a helper of sailors seems more like something loaned from the Norse Njörðr.

In 1728, a man named Påhl Hindrikinpoika Paskoja was accused of witchcraft in court in Pyhäjärvi. He had to sing runic incantations during the trial, including the origin of fire, where Ilmarinen and Väinämöinen struck fire by forging. He explained that Ilmarinen was a god people used to worship, while Väinämöinen was something which lived in a mountain.