A Stockholm district court in December found Rachid Zoghlami guilty of rape and sentenced him to two years and eight months in jail, after he penetrated his girlfriend with his fingers to determine whether she had been unfaithful.
But a court of appeal ruled that the 28-year-old's actions should be viewed as coercion rather than rape since they weren't driven by "sexual intent", and reduced his sentence to 18 months behind bars.
Zoghlami, who holds dual Finnish-Tunisian citizenship, admitted to being jealous but said he used no violence against girlfriend Carina Johansson, 30, when using two fingers to "search for sperm".
The case has attracted widespread media attention, and the appeals court ruling prompted calls for the Swedish law to be changed to broaden the definition of a sexual crime.
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Today, the Supreme Court ultimately ruled it was a question of rape.
Sweden's Supreme Court only accepts cases if they set a legal precedent, or if a lower court has committed a grave judicial error during a trial.
"If a man forces a woman to accept that he penetrates her with his fingers, then the act is clearly of a sexual nature and aimed at violating her sexual rights. Therefore this is a punishable sexual act," the court wrote in statement.
The Supreme Court sent the case back to the appeals court to determine Zoghlami's sentence.