An Oslo court ruled that Marius Borg Hoiby, who joined the royal family when his mother Mette-Marit married Haakon in 2001, was guilty of two counts of rape including one in the basement of the crown prince's home.
He was acquitted of two other rape charges.
Hoiby, 29, had pleaded not guilty to the most severe accusations against him, including those of rape, while admitting to some lesser ones, and can appeal the verdict.
Prosecutors had sought seven years and seven months in prison.
The case has exposed Hoiby's drug addiction, self-made videos of sexual encounters and more than 800 electronic messages entered into evidence.
"The court finds it is proven she was not able to resist the action," Oslo District Court Judge Jon Sverdrup Efjestad said of the rape at the crown prince's home, while reading the verdict.
Hoiby watched the verdict via video link from prison but could not be seen or heard in the courtroom.
VICTIM TEARFUL IN COURT
Only one of the women accusing him of rape was in court to hear the verdict. She cried after the judge said Hoiby was guilty of raping her and dabbed her eyes with a tissue her lawyer gave her.
Hoiby's actions have further dented the popularity and once very positive public image of the royal family.
It coincided with Crown Princess Mette-Marit's apology for "poor judgement" in maintaining contact with the late U.S. sex offender Jeffrey Epstein after he was convicted in 2008.
A Norstat survey out on February 21 - during the trial - showed a fall in the number of Norwegians favouring keeping the monarchy to a record low of 60%, from 70% in January, and a rise to 27% from 19% in those wanting a different system of governance.
In May, the royal family recovered somewhat in popularity, with 64% polled by Norstat supporting the monarchy and 23% wanting a different system of governance.
The verdict was delivered amid difficult personal circumstances for Mette-Marit, Hoiby's mother, who needs a lung transplant for pulmonary fibrosis.
