The memorial to Benjamin Hermansen, a Ghanaian boy stabbed in 2001 by neo-Nazis, has been tagged with the text "Breivik was right" in the Oslo district of Holmlia.
The Oslo Police believe it is serious as it comes 10 years after the mass shooting on the island Utøya that left 77 dead, the bloodiest peacetime killing in Norway, and have opened an investigation into the case.
"We are taking it seriously since it is happening right now. We are almost 10 years away from the incidents on Utøya and since reference is made to Breivik in the damage that has been done, we think it is natural that we do further investigations", Police Inspector Egil Jørgen Brekke told national broadcaster NRK.
The police have asked for tips from the public and are working to find out whether any surveillance cameras nearby filmed the incident.
The incident sparked outrage among Norwegian politicians and community workers alike.
Jonas Gahr Støre, the leader of the Labour Party, whose youth wing Breivik had targeted, called the vandalism an extreme act.
"The intelligence service, the police, and the security services cannot fully protect us against such attitudes. We must tackle it together, it is a societal issue", Støre told the national broadcaster.
Fellow party member and City Council leader Raymond Johansen called it "disgusting".
Conservative Prime Minister Erna Solberg said it is "terrible" to see the memorial being tagged just before 22 July.
"I am sad and angry, and this shows how important it is that we stand up against racism and hate speech every single day", she said.
Beatriz Jaquotot, a board member of the Benjamin Memorial Fund, called the deed reprehensible.
"There have been both tears and anger earlier today", she said, praising the local community for quickly washing the tag away. "It's fortunately gone now".
Benjamin Hermansen, 15, was killed at Holmlia in Oslo in January 2001. The murder was found to be racially motivated, and three members of the neo-Nazi group Boot Boys were subsequently sentenced to prison for complicity in the murder. Throughout the entire country, marches were organised in protest against the murder. In 2003, the annual Benjamin Prize was founded in Hermansen's memory.
Notably, US singer Michael Jackson dedicated his 2001 album "Invincible" to Benjamin Hermansen.
In 2011, Anders Behring Breivik became the deadliest mass murderer in Norway's history, pulling off a dual attack against Oslo's government quarter and the Labour Youth Camp on Utøya Island, killing 77 and wounding over 150.
Prior to the attacks, which left Norway's national psyche deeply traumatised, Anders Breivik issued a manifesto voicing fierce opposition to multiculturalism and Islam and blamed feminism for what he called Europe's "cultural suicide". Diagnosed with narcissistic and antisocial personality disorder, Breivik has since been serving Norway's harshest sentence of 21 years in solitary confinement.