Police said on Friday they would bring new charges against Mullah Krekar, an Iraqi Kurd and co-founder of Ansar al-Islam, a group which Washington says has ties to al-Qaida.
Krekar was arrested at his home in Oslo around midday, police said.
The Iraqi has had refugee status in Norway since 1991, and has denied links to "militant groups".
But senior police official Erling Grimstad, who heads the investigation, said police would ask an Oslo court on Saturday to order that Krekar be detained for four weeks while they prepare a case against him.
"We have a new charge in the case against Mullah Krekar," Grimstad said, although he declined to elaborate and said police would also ask for a closed-door court hearing.
Police first arrested Krekar on 20 March, the day US-led forces began an invasion to topple Iraqi leader Saddam Hussein.
Krekar was charged under anti-terrorism legislation of links to a terrorist organisation and of threatening violence.
An appeals court ordered his release in April and the charges were finally dropped in July when Grimstad acknowledged that police did not have enough evidence.
The US accuses Ansar al-Islam of
being behind resistance attacks
Krekar's attorney Brynjar Meling said on Friday that he had been informed of the arrest, but had not yet met his client and did not know what the new charges were.
"I am not sure exactly what they have connected him to," Meling said on his way to meet Krekar.
The US air force targeted Ansar's camps in northern Iraq during the war.
US officials have also named it as one of the groups behind resistance attacks that have killed more than 200 American soldiers since major combat was declared over in May.
Last year Washington asked Oslo to extradite Krekar to Jordan to face criminal charges there. But in November Norway rejected a Jordanian request for his extradition to face drug trafficking charges.