Wednesday's decision cited in part the actions of District Judge G. Todd Baugh (baw) of Billings, who suggested the young victim shared responsibility for her rape because she had some control over the situation.
Justices said a new judge must re-sentence defendant Stacey Dean Rambold.
Rambold has been free since completing his sentence last fall. Prosecutors for the state say he should serve a mandatory minimum of four years prison.
Rambold's attorneys had argued that the original sentence was appropriate.
A disciplinary complaint against Baugh from the Judicial Standards Commission is pending with the state Supreme Court. Justices say that will be dealt with separately.