"We are satisfied that the arms decommissioning represents the totality of the IRA's arsenal," said John de Chastelain, a retired Canadian general who since 1997 has led efforts to disarm the outlawed IRA.
The material included ammunition, rifles, machine guns, mortars, missiles, handguns and explosives, de Chastelain told a news conference on Monday.
All the weapons were rendered "permanently inaccessible or permanently unusable", he added.
The IRA permitted two independent witnesses, a Methodist minister and a Roman Catholic priest close to Sinn Fein leader Gerry Adams, to view the secret disarmament work conducted by officials from Canada, Finland and the US.
Questioned by reporters, de Chastelain said he could not be absolutely certain that every IRA weapon had been disposed of, but he said he believed the IRA was sincere in saying it had handed over the whole arsenal.
He also said the amount was consistent with police and army estimates of the IRA's arms holdings.
The breakthrough should overcome the biggest stumbling block in Northern Ireland's peace process since Britain opened negotiations with Sinn Fein, the IRA-linked political party, in December 1994.