An eight-member jury picked Ivan over Jan Cvitkovič's Družinica (The Basics of Killing) and Družina (The Family), an observational documentary by Rok Biček.
The jury said Ivan was the most sophisticated of the three films, "shocking the viewer with the first scene and telling a brutal story" of the main heroine.
The film focuses on the story of Mara, a young mother who realises she needs to side with her son Ivan and stand up against his wealthy father, to whom she feels an obsessive attachment.
The jury said that Maruša Majer, who plays Mara, leads the viewer through her transformation from a dependant, naive, lost girl to a women who is ready to face her childhood demons and take responsibility for herself and her child.
"The narrative is upheld with convincing acting by other actors, nonintrusive make up art that makes everything seem real, and photography that intimately leads us through the film and allows us to experience the heroine from up close."
Ivan was the main winner of the 20th Festival of Slovenian Film, receiving eight awards, including for best feature live action film, best screenplay and best leading actress.
The film has also won several other awards, including the Danny Lerner Award for best feature film at the regional LET'S CEE Film Festival in Vienna this spring.
Ivan was produced by Staragara together with Tramal Films and Croatia's Propeler film.
The nominees for the Best Foreign Language Film Oscar will be picked by the Motion Picture Academy.