Sif is a character appearing in American comic books published by Marvel Comics. Created by Stan Lee and Jack Kirby, the character first appeared in Journey into Mystery #102. She is based on the Norse goddess Sif. As an Asgardian warrior and lover of Thor, Sif often accompanies him into battle. She has also battled alongside Balder, who has developed an unrequited attraction to her, as she never shows affection for anyone but Thor and certain individuals who have proved worthy to wield his hammer, Mjolnir, such as the noble alien warrior Beta Ray Bill, and the mortal Eric Masterson.

In Norse mythology, Sif is a golden-haired goddess associated with earth. Sif is attested in the Poetic Edda, compiled in the 13th century from earlier traditional sources, and the Prose Edda, written in the 13th century by Snorri Sturluson, and in the poetry of skalds. In both the Poetic Edda and the Prose Edda, she is known for her golden hair and is married to the thunder god Thor.