In Finnish mythology, the Sampo (pronounced [ˈsɑmpo]) or Sammas (pronounced [ˈsɑmːɑs]) is a magical device or object described in many different ways, constructed by the blacksmith, inventor and originally the sky god Ilmarinen, and which brings riches and good fortune to its holder, akin to the horn of plenty (cornucopia) of Greek mythology. A central myth in Finnish mythology is the idea that the sampo was once in Pohjola but a group of heroes attempt to steal it. Sammas, as something at the center of the world, also exists in Estonian mythology.

The Sampo or Sammas has multiple other names in runic songs as well, including Samppu, Samppi and Kirjokansi (pronounced [ˈkirjoˌkɑnsi]).

The Sampo has been interpreted in many ways: a world pillar or world tree, a compass or astrolabe, a chest containing a treasure, a Byzantine coin die, a book, a decorated Vendel period shield, a Christian relic, etc. In the Kalevala, compiler Lönnrot interpreted it to be a quern or mill of some sort that made flour, salt, and gold out of thin air. The world pillar/tree hypothesis was argued for by figures such as theosophian Pekka Ervast, historian of religions Uno Harva and the linguist Eemil Nestor Setälä in the early 20th century.

In Estonian runic songs, the Sammas, an oak or an ash (akin to the Yggdrasil) is something which stands at the center of the world. According to the archaeologist Elena Kuz'mina, the Sampo mill myth originates from the Indo-European skambhá ("support, pillar, column"), and was borrowed into Finno-Ugric. In the Atharvaveda, the skambhá is a creature that supports the universe, analogous to the World Tree – the Sampo has been claimed to be the Finnish equivalent of the world tree. At the top of this tree is the kultakätkyt ("golden cradle"), which according to Forest Finn tietäjä Kaisa Vilhunen is a veśkoaŕ ("rainbow"), which works as a bridge between heaven and earth.

The world pillar or tree is attached to the center of the sky, turning it around, while also working as a path between the different layers of the world. Vilhunen gave a detailed explanation of the Sammas which supports the theory of its nature as the world pillar. She described it as a golden pillar with a golden head and a copper foot. The golden head is Polaris, which attached the Sammas to the sky. The pillar went down deep underground like the roots of a tree and was attached to a copper mountain there. The dome of the sky spun around Polaris, which Vilhunen described as the Sammas "grinding". According to her, the root has three guardians: Tuokkoi, Pajas and Ruoskakup ("whip guy"). They make sure no one can dig up the gold at the root of the pillar so it won't snap. The identities of these guardians are a mystery, but Pajas could mean Ukko: the Forest Finnish Sammas myths originate from Savonia, where Ukko also has the name Pajainen. Jenuveeva (Genevieve) sat at the top of the pillar, at the center of three branches.

The mill-like qualities of the Sampo connect it to the Grotti mill from Norse mythology, which like the Sampo is a source of wealth and everything good. Martti Haavio saw the two as different branches of the same myth of creation. He also connected it to myths from Ancient Indian literature. He thought Sampo's name comes from *tšampa, which would also be the origin of sammakko ("frog"). Connection to Ancient India holds up, as Sampo is possibly a loan from Indo-Iranian languages, but the frog theory is not supported in contemporary research.

For runic singers, the Sampo was a representation of growth and its mythic origin, and initially not an object. Ilmarinen was able to forge the Sampo because he is a god, the one who had also created the sky. Anna-Leena Siikala thought that the concepts of the theft of the Sampo and its mill-like qualities are a sign of influence born out of contacts with the Norwegians. A runic song from Satakunta mentions a smith creating a noitakone ("witch/shaman machine"), the nature of which is a mystery, unless it's a Sampo-like magical object.

Matti Kuusi described the purpose of the Sampo myth as a ritual song of the birth of agriculture.