The right-wing extremist killed mainly teenagers in his rampage on July 22, 2011, hunting down participants at a camp of the Labour Party's youth wing (AUF) on the tiny heart-shaped island in the middle of a lake.
Determined to reclaim possession of the site, the youngsters -- including a handful of survivors -- are holding their annual camp from Friday to Sunday.
The atmosphere was relaxed as AUF head Mani Hussaini told the delegates in his opening speech: "It's good to be back home."
In his only direct reference to the carnage of four years ago, Hussaini said: "July 22 will forever be part of Utoya's history... but this day is also going to go down in Utoya's history."
NATO Secretary General Jens Stoltenberg, who was prime minister at the time of the attack, was attending the camp and tweeted: "Great to wake up at Utoya, and to be together with so many engaged young people."
Many of the teenagers arrived on Utoya on Thursday, with many pitching their tents near the cafeteria, a poignant symbol of the massacre as Breivik killed 13 youths there. Bullet holes can still be seen in the building.
Before the seminars and speeches began on Friday, the teenagers held high-spirited games of football or volleyball, although armed police guards kept a careful watch. Two police boats are guarding the waters around Utoya.
Many of the delegates wore T-shirts bearing the party slogan "Working Class Hero".
Hussaini told reporters taken to visit the island earlier this week that Utoya was "a meeting place for young activists, a political workshop, a place for culture, sport, friendship, and not least, love".
"Utoya is also the site of the darkest day in Norway's peacetime history," he added. "Utoya will always be the place where we will remember those we lost, but reclaiming Utoya for the summer camp is about not letting the dark history overshadow the light," he said.
Breivik's shooting spree lasted an hour and 13 minutes, as he methodically stalked and killed so many of the 600 up-and-coming leaders of Labour, Norway's dominant political party, which he blamed for the rise of multiculturalism.
Trapped on the island of just 30 acres (12 hectares), the campers had nowhere to go, some of them throwing themselves into the surrounding chilly waters.
Just before the shootings, Breivik had killed eight people with a bomb that exploded near the government headquarters in Oslo, some 40 kilometres (25 miles) away.
Norwegian authorities were harshly criticised for their lack of preparedness and their slow response to the attacks at the time.
The wooded island has received a facelift ahead of the reopening. New buildings have been built next to the old ones, which have been carefully renovated.
A little further away, a memorial entitled "The Clearing" has been installed in the woods: a giant steel ring suspended from the evergreens bears the names of 60 of the 69 victims.
But in a sign that the wounds are far from healed, nine families did not want their loved ones' names to appear on the ring.
Some in Norway feel it was too early, even disrespectful, to hold a summer camp at the scene of the tragedy. But 22-year-old survivor Ole Martin Juul Slyngstadli was not one of them.
"There are of course a lot of emotions linked to the scene but I focus on the positive ones," he told AFP.
"For me, it's important to reclaim the island," he said. "We've found the balance between the duty of honouring the memory (of the victims), dignity and going back to normal," he added.
While Breivik said his goal was to wipe out the next generation of Labour politicians, his attacks appear to have had the opposite effect. AUF's membership has soared by almost 50 percent since the massacre to reach just under 14,000. Among the new members are children of migrant
families, who represent everything Breivik hated.
"I felt it was important, that I wanted to make a contribution," said 22-year-old Joel Gianni, whose parents are from Eritrea, and who says he joined the party after the attack.
He was attending the Utoya camp for the first time.
"I had my doubts. I thought the ambiance might be weighed down by the memories of July 22 but that's not the case: it's there a bit, of course, but not too much."
Breivik is in solitary confinement serving a 21-year prison sentence, which can be extended indefinitely as long as he is considered a danger to society