Thjálfi (Old Norse) or Thjelvar (Old Gutnish) is a person (or two) from Norse mythology who appear(s) twice in Snorri's Edda and once in the Gutasaga.

Edda

Snorri relates that when Thor visited the boy Thjálfi's family, Thjálfi crushed the bone of one of Thor's butchered goats. When Thor resurrected the goat, it could not walk properly. To atone this, the boy and his sister Röskva had to serve Thor as his thralls. In the story of Thor and Mökkurkalfe , Thálfi helps Thor to fight two giants.

Gutasaga

In the Gutasaga, Thjelvar is an adult and arrives at a magical island which sunk every night and resurfaced every day. He lit a fire on the island and from then on it sank no more. Thjelvar's son Hafthi married the girl Huítastjerna (White Star) and they became the ancestors of the Gutar (the Gotlanders).

Tjelvar's Grave Image is a ship setting, which according to legend is the grave of Thjelvar, and it is dated to ca 750 BC.