Hours from the first appearance in court of mass killer Anders Behring Breivik, palace spokeswoman Marianne Hagen told AFP that 51-year-old off-duty policeman Trond Bernsten died during Friday's massacre.
She did not give further details, but the Verdens Gang daily, citing witnesses, said he had tried to arrest the gunman after ensuring his son was safe.
"But the 32-year-old gunman did not hesitate for one second in his murderous enterprise, and killed the unarmed police officer," the newspaper wrote.
According to press reports, Bernsten was on the island as a private guard for the gathering of hundreds of members of the main ruling Labour party's youth wing.
Bernsten was the son of the second husband of Mette-Marit's mother. The princess entered the royal family in 2001 after her marriage to crown prince Haakon.