They believe he was in a psychotic state during the twin attacks on 22 July that led to the deaths of 77 people and injured 151.
He was also insane during the 13 interviews the two psychiatrists held with them, a news conference heard.
Breivik admits carrying out the attacks but has pleaded not guilty to charges.
He has previously said the attacks were atrocious but “necessary”.
The two psychiatrists, in their report, concluded that he lived in his “own delusional universe where all his thoughts and acts are guided by his delusions”.