"I can confirm that he has changed his name, it's official," Oystein Storrvik told AFP, confirming reports by the Norwegian newspaper Verdens Gang (VG).
Asked why Breivik had decided on the name change, Storrvik said: "I do not want to disclose the content of our discussions."
In July 2011 Breivik, disguised as a police officer, tracked and gunned down 69 people, most of them teenagers, at a Labour Party youth camp on the island of Utoya, shortly after killing eight people in a bombing outside a government building in Oslo.
He has never expressed any remorse for committing the worst atrocity in Norway's post-war history. He said he killed his victims because they embraced multiculturalism.
Before proceeding with the attacks, he circulated an ideological "manifesto" signed under the name Andrew Berwick.
A search in the Norwegian business register confirms that Breivik Geofarm, an agricultural firm created by Breivik to obtain fertilisers used to make a bomb, is now registered in the name of Fjotolf Hansen.
While Hansen is a very common surname in Norway, Fjotolf is rarely used, if ever.
The now 38-year-old inmate is serving a 21-year prison sentence that can be extended indefinitely.
Breivik has complained about his isolation from other inmates for safety reasons since his arrest in 2011, and sued the Norwegian state over his prison conditions.
His lawyer said on Thursday that he would take the case to the European Court of Human Rights after exhausting all legal options in Norway where the Supreme Court refused to hear his case.