Kullervo (Finnish pronunciation: [ˈkulːerʋo]) is a hero in Finnish and Estonian mythology. He is often called a son of Kaleva. He also appears as an ill-fated character in the epic Kalevala by Elias Lönnrot.

Runic songs of Kullervo mention him as a son of Kaleva. Sometimes, his father is called Kalervo, but this is a later variation which morphed from Kaleva. Ingrian and Karelian runic songs further tell of the fight between two families, the other being the family of Kaleva or Kalevainen, who ruin each others fields and possibilities to survive. Kalevainen and his family were exterminated but an unborn child survives and continues the family feud, bakes a stone inside the bread of a mean mistress of a house, and ends up committing incest. In a different runic song from Ilomantsi, after killing Kaleva, the enemies try to kill his son by throwing him into a large bonfire. However, he had the ability to control fire. To avenge his family, he asks Ilmarinen to forge him a sword and goes to war. Other songs tell he was later defeated on the battlefield. In a runic song written down in the 1700s from an unknown location, it is Kullervo himself who is killed (on the battlefield) but has an unborn child who is later thrown into fire, who is able to control fire.

In runic songs from Estonia, Kuller Kalevi poega ("Kuller, son of Kalev") creates himself an artificial wife from gold or wood. However, the golden maiden is cold to the touch. Kullervo is not always the protagonist of this story, as it might also be Lemminkäinen, an unnamed smith or Kalev himself. In runic songs from Finland, the smith is usually Ilmarinen or Väinämöinen. The songs ends with the commandment that one must not make himself a wife out of gold and silver.

In a runic song from Kainuu, a cowshed snake is born out of Kullervo's war spear snapping into two. In this context, his name appears as Istervo in North Karelia, Istervö in North Karelia, Ladoga Karelia and North Savo, Istori in South Savo, Lispervo in Ladoga Karelia, Lisperi in North Karelia, and Histervo in North Karelia and in a runic song from an unknown location.

In a runic song from South Savo, Kullervo asks, as he is going to war, if his father and mother would cry after hearing he has died. His father says no, as he would simply make another, better son. However, his mother says she'd cry so much it would melt all snow and turn earth green. When he gets the message that his mother has died, he doesn't care, thinking he could make a better mother for himself from seeds, twigs and leaves. In a runic song from Central Finland, when he gets the message he similarly doesn't care, being too busy fighting in the war.

In some runic songs, an unnamed "beautiful son of Kaleva" is sold as a slave to a smith in Karelia. When he is asked to look after a child, he kills the child with a disease. When he is asked to build a fence, he ties full grown spruces together with snakes and lizards. When he is asked to herd cattle, his mistress bakes a stone into his bread. This angers him, and he summons bears and wolves to kill the mistress. In many regions, the protagonist in question is identified as Kullervo, but in Ostrobothnia and Kainuu, the protagonist is Soini.

Growing up in the aftermath of the massacre of his entire tribe, he comes to realise that the same people who had brought him up, the tribe of Untamo, were also the ones who had slain his family. As a child, he is sold into slavery and mocked and tormented further. When he finally runs away from his masters, he discovers surviving members of his family, only to lose them again. He seduces a girl who turns out to be his own sister, having thought his sister dead. When she finds out it was her own brother who seduced her, she commits suicide. Kullervo becomes mad with rage, returns to Untamo and his tribe, destroys them using his magical powers, and commits suicide.

At the end of the poem the old sage Väinämöinen warns all parents against treating their children too harshly.