"She wasn't able to describe either violence or threats, and therefore it's not rape according to the letter of the law," prosecutor Barbro Brännlund told the local Nya Wermlands-Tidningen (NWT).
The woman reported the rape to police in Karlstad about a year and a half ago, explaining that an acquaintance had forced her to have sex with him against her will.
She told the man "no" repeatedly and did her best to hold her legs together, the woman told investigators.
But when the man refused to stop, the woman "shut down", allowing the man to carry out the act.
As there was no forensic evidence in the case, it became of matter of one's word against the other's.
While the court explained that it found the woman's version of events credible, the man was ultimately freed because it "hadn't been shown beyond a reasonable doubt" that the man had forced the woman to have sex with him.
Brännlund is concerned that women in Sweden may not understand what's required for to secure a rape conviction.
"Even if a no is a no, it's not enough to just say it for it to be rape," she told the paper.
"In my mind, there's no doubt whatsoever that the woman perceived this as an attack. But the man never realized that."
The case almost never made it to court because evidence was so thin and the prosecutor has signalled she's unlikely to appeal the court's ruling.