In a letter obtained by the German publication Der Spiegel, Breivik praised Beate Zschape, the only surviving member of the National Socialist Underground, for her alleged crimes and said she could be considered "a courageous nationalist resistance hero" who tried to stop multiculturalism and "the Islamization of Germany."
Zschape, 37, is accused of participating in the deaths of 10 people and is awaiting trial at a prison in Cologne, where Breivik sent the letter in May, Der Spiegel said Monday.
She was formally charged last week.
"If you end up being convicted due to overwhelming evidence, then you might reach a point where you may flag your political motives," he wrote in the three-page letter authorities intercepted before it reached Zschape.
"If it is clear that you are indeed a militant nationalist that chose to contribute this way, [then] you will be regarded in a lot of people's eyes as a courageous nationalist resistance hero who did everything you could and sacrificed everything to stop multiculturalism and the Islamization of Germany."
The NSU is suspected of killing a police officer and nine other people, mainly small-business owners of Turkish and Greek heritage. The group also was accused of staging two bomb attacks and 15 armed robberies during the past 12 years.
In August, Breivik was sentenced to 21 years in prison for killing 77 people July 22, 2011, eight in a bombing in downtown Oslo and 69 attending a Labor Party youth summer camp on nearby Utoya Island.
The sentence, the longest prison term possible for murder and terrorism under Norwegian law, includes the possibility of indefinite extension if a judge determines the self-proclaimed, far-right anti-Muslim extremist still is threat to society.