The verdict, in line with what Breivik himself wanted,brings to an end a spectacular 10-week trial for the twin attacks thattraumatised normally tranquil Norway and shocked the world.
Breivik killed eight people in an Oslo blast and took 69 more lives, mostly teenagers', in a shooting frenzy at an island summer camp on July 22, 2011.
"The ruling is unanimous," presiding judge Wenche Elizabeth Arntzen told the court.
"He is sentenced to prison for 21 years, with a minimum of 10 years," she added. Under Norwegian law the sentence could be extended.
Breivik, wearing a dark suit with a white shirt and a grey tie, smiled as the verdict was read out in court.
Survivors of the Utoeya island massacre took to Twitter immediately to comment on the sentencing, with Emma Martinovic tweeting: "YEEEEEEESSSSSSSS!!!"
And Viljar Hansse, who took a bullet to the head in the massacre, tweeted: "Finished. Period."
Breivik has previously said he would not appeal a prison sentence, as he wanted to be found sane so his Islamophobic ideology would not be considered the rantings of a lunatic.
Norway's penal code does not have the death penalty or life in prison, and the maximum prison term for Breivik's charges is 21 years. However, inmates who after that are still considered a threat to society can be held indefinitely.
The 33-year-old loner, who made a right-wing salute in court after his handcuffs were taken off, had confessed to the attacks, seeing himself as a Nordic warrior against Europe's "Muslim invasion" and against all those who promote multiculturalism.
