Barbro Brannlund told the court that the woman failed to describe either violence or threats and therefore wasn't legally a victim of rape, Sweden's The Local.se reported Wednesday.
Brannlund said she is concerned Swedish women may not understand what is required to secure a rape conviction.
The woman reported the rape to police in Karlstad in 2011, saying an acquaintance had forced her to have sex with him against her will.
She said she told the man "no" repeatedly and tried to hold her legs together but the man eventually prevailed.
The court said it found the woman's testimony credible but was forced to rule against her because "it hadn't been shown beyond a reasonable doubt" that the man forced the woman to have sex with him.