In a long-awaited statement, the paramilitary group ordered its units to fully decommission, ending more than 30 years of violence. It would henceforth pursue a united Ireland through only "democratic, peaceful means," it said. The statement takes effect at 4 p.m. Thursday.
British Prime Minister Tony Blair welcomed the statement, which he said was "of a different order" to anything before.
"This may be the day when finally after all the false dawns and dashed hopes, peace replaces war, politics replaces terror on the island of Ireland," he said.
The group must fully disarm as soon as possible, he said, and resumption of a devolved power-sharing executive in Northern Ireland should follow.
The statement followed an appeal by Sinn Fein Leader Gerry Adams in April that the organization end its armed campaign and engage in the democratic process.
The IRA has been blamed for some 1,800 killings during Northern Ireland's Troubles.