Sufyan Daud, a former separatist commander, said on Tuesday "the Acehnese national army, or the armed wing of the Free Aceh Movement, has demobilised and disbanded".
The Aceh national army is now part of civil society and will work to make the peace deal a success, he said, adding that the action takes effect immediately.
The move paves the way for the group, which has fought a bloody insurgency against government troops for almost 30 years, to transform itself into a political party that is expected to win strong support at provincial elections planned for April.
The announcement came shortly after separatist representatives met Susilo Bambang Yudhoyono, the Indonesian president, in Banda Aceh, the capital of Aceh province that was worst hit by the Indian Ocean tsunami one year ago.
The devastation wrought on the province by the disaster spurred the peace deal, under which the government will withdraw troops and grant the Acehnese special autonomy in exchange for the separatists laying down their guns.
The two sides met on Monday as the world marked one year since the tsunami crashed into the coastlines of a dozen countries on the Indian Ocean's rim, leaving at least 216,000 dead or missing - more than 156,000 of them in Aceh.
Susilo, in a speech marking the anniversary, said the tsunami had afforded a "golden opportunity" to end the conflict and suggested that the peace deal was "an example of how a new hope for peace can emerge out of the ruin or destruction".
Susilo: The tsunami offered an
opportunity to end the conflict
At a news conference earlier on Tuesday, Susilo did not mention the separatists' impending announcement.
Indonesian authorities and the separatists - known as GAM, an acronym of the group's Indonesian name - had been moving towards peace talks before the tsunami, but the disaster forced both sides to focus on ending their war.
Peace talks opened in January and were successfully wrapped up by July.
The separatists gave up their demands for a referendum such as the one that ended Indonesian rule in East Timor in 1999, while the government promised them broad autonomy and allowed them to take part in regional elections that they are expected to win overwhelmingly.
So far, both sides have stuck to the agreement.
"The Aceh national army is now part of civil society, and will work to make the peace deal a success"
former GAM commander
The separatists have handed over their weapons to a European Union-led observer mission, while more than 24,000 government troops have pulled out of Aceh.
The last are due to leave by the end of the year.
Aceh has a long history of opposing outside rule. The current rebellion, in which at least 15,000 people have died, began in 1976.
A previous attempt to end the bloodshed collapsed in 2003, after the Indonesian military expelled foreign observers and restarted combat operations against the separatists.