The Oslo district court found the 37-year-old's treatment in prison violated Article 3 of the European Convention on Human Rights, prohibiting "inhuman or degrading treatment," and ruled that his conditions must be eased.
The court also ordered the government to pay legal costs of 331,000 kroner ($40,600) for the right-wing extremist, who killed 77 people
in a shooting rampage and bombing attack in 2011.
Norway has the right to appeal the ruling. It has not announced whether it intends to do so.
The court dismissed Breivik's claim that the government had violated Article 8 of the European Convention on Human Rights, which guarantees respect for "private life" and correspondence.
The ruling outlined areas of concern in regard to the conditions of Breivik's confinement, which, taken as a whole, constituted a breach of his rights.
These included the duration of his isolation, and inadequate consideration of the mental impact of the regime. It also said the routine nude checks Breivik had to go through were not sufficiently justified from a security perspective.
But it did not give concrete directives on how the conditions should be changed.
Breivik: Conditions 'sadistic'
The suit was heard over four days last month
inside a gymnasium at Skien prison, which was temporarily converted into a courtroom.
Appearing in public for the first time since his trial, Breivik gave testimony during the suit, alleging that his isolation in prison constituted a "sadistic" attempt by Norwegian authorities to kill him.
Since his arrest, he has been separated from other inmates, and virtually his only visits have been with professionals, who meet with him separated behind a glass screen.
His incoming and outgoing mail is also censored to prevent him from building far-right networks and inciting sympathizers to violence.
Observers expressed concern that Breivik -- who gave a Nazi salute on his first day in court
-- was using his court appearance as a platform to publicize his extremist ideology.
Breivik's killing spree on July 22, 2011, was the deadliest attack in Norway since World War II. Eight people were killed when a bomb he planted detonated in Oslo before he methodically shot to death 69 young people at a Labor Party youth camp on Utoya island. He blamed the party for the rise of multiculturalism in Norway.
Bjorn Ihler, one of the survivors of the Utoya massacre, said on Twitter that the ruling was a "sign we have a working court system, respecting human rights even under extreme conditions."
"Our best weapon in fighting extremism is humanity. The ruling in the Breivik case shows that we acknowledge the humanity of extremists too," he wrote.