In Norse mythology, Sif (/siv/,"relative") is a goddess of the Aesir, wife of Thor and mother of Thrud, Ull and Modi. Thor is not the father of Ull.

Sif was not the mother of Magni; this was Thor's son with Jarnsaxa.

She has hair of gold which grows just like natural hair, made by the dwarves called "sons of Ivaldi" (Dvalin). As wife of the god of storms and rain, she was believed to help crops grow, and her hair's color may have represented ripe wheat. Loki cut off Sif's hair as a prank, but when Thor found this out he forced Loki to promise to have golden hair made by the dark elves. Loki uses this event to taunt Sif and, indirectly, Thor in Lokasenna.

Loki succeeded in having such hair made, as well as Freyr's ship Skidbladnir and Odin's spear Gungni, all made by the same dwarf Dvalin.