Rune Øygard (54), who stood down last year as mayor of Vaga, north of Oslo, has always denied the charges.
But an appeals court earlier held it was proven that he had had some 50 sexual encounters with the girl, who is now 17, over a two-year period.
Aage Thor Falkanger, one of the judges in the case, wrote in his judgement that "a number of aggravating circumstances" meant that the the normal punishment of six months in prison should only "give limited guidance".
"This is a historically strict judgment, as I see it. It is totally without basis in case law and heralds a new trend when it comes to sentencing," Øygard's defence lawyer Mette Yvonne Larsen told NTB.
Øygard’s sentence was slashed from the punitive four years given by the appeals court. Larsen had argued for a still shorter sentence, to take into account the two years of massive media scrutiny her client had had to endure.