The finding by the two forensic psychiatrists will help determine whether Breivik is sentenced to prison or psychiatric care. Prosecutor Svein Holden says the report shows Breivik was "psychotic" during the attack.
If that assessment is upheld by the court then Breivik cannot be sentenced to prison for the attacks.
"The conclusion is ... is that he is insane," Holden told a news conference. "He lives in his own delusional universe and his thoughts and acts are governed by this universe."
The two psychiatrists, Synne Serheim and Torgeir Husby, delivered their finding to the Oslo district court on Tuesday morning.
"We have no doubt when it comes to our conclusions," Husby told reporters as he submitted the report.
"It was a lot of work, demanding," Husby said, adding: "He has cooperated well."
Breivik, 32, was suffering from a "psychosis" – a mental state that could alter his judgment leading up to and at the time of the attacks, VG writes.
Their report still needs to be examined by a legal medical commission to ensure that it fulfils all the professional requirements.
The court will ultimately determine whether Breivik can be held accountable for his actions, but it is common practice in Norway for courts to follow expert recommendations.
Breivik has admitted setting off a car bomb outside the government offices in Oslo on July 22, killing eight people, before going on a shooting rampage on the nearby island of Utoya where the ruling Labour Party's youth wing was hosting a summer camp.
Sixty-nine people, mostly teens, died in the shooting massacre and police have said they found 186 empty shell casings strewn around the island.
In a manifesto he published on the internet just before the attacks, Breivik said he was on a "crusade" against Islam and professed his hatred for Western-style democracy, saying it had spawned the multicultural society he loathed..
