Plaid Cymru will enter government for the first time in a momentous Welsh assembly coalition with rivals Labour.
The day after Labour voted by a big majority to share power, Plaid followed suit by 225 votes to 18, or 92%.
Plaid leader Ieuan Wyn Jones will be deputy to First Minister Rhodri Morgan, and other Plaid AMs join the cabinet.
It also means that eight years after devolution began, nationalists will be in government in Wales, Northern Ireland and Scotland.
Hundreds of Plaid members were at the national council meeting in Pontrhydfendigaid, Ceredigion, the morning following Labour's crucial vote in Cardiff.
Plaid ratified the One Wales document, as Labour had done by 78.4% to 21.5% at a special party conference in Cardiff on Friday, even though a number of Labour AMs and MPs were strongly opposed.
Mr Jones said he was "absolutely delighted" and "very proud to be the first Plaid leader to be taking the party into government."
He said: "The decision we have come to today has been a long time in the making. It may have taken us nine weeks to negotiate a new government for Wales but it has taken Plaid Cymru 82 years to reach this point."
The Plaid leader said the assembly government should represent "all parts of Wales wherever they live and whatever their background," and his party had to ensure the deal was "not in the interests of any one party but putting the interests of the people of Wales first."
He described it as "a historic moment in the life of our party, in the life of our politics and the life of our nation".
'Deliver on policies'
Mr Jones declined to comment how many cabinet seats Plaid will want. At present there are seven cabinet members, and the law says that there can be a maximum of 14 - although two of these would be the first minister and the legal adviser the consul general, who need not necessarily be an AM.
First Minister Rhodri Morgan and Ieuan Wyn Jones struck the deal
He said the decision was "at the very top" of the events in Plaid's history since it was formed in 1925.
"I've always wanted to see Plaid as a party of government," he said. "If people vote for you in elections, they want to see you deliver on policies, and now for the first time we have a chance."
Last week Welsh Labour leader and First Minister Rhodri Morgan and the Plaid leader agreed in principle to the agreement, which emerged after Labour failed to gain a majority in May's assembly election.
Most AMs from both parties have also supported the document, as have their national executives.
But there are also some notable opponents, including former Labour leader Lord Kinnock, ex-ministers Paul Murphy and Don Touhig, and current minister Kim Howells.
Before Plaid voted, Neil McEvoy, a Plaid councillor in Cardiff said: "I have not spoken to anyone who is against it. It's the only option really.
"We just have to be careful with Labour, possibly have an exit strategy if we must".
After the Plaid decision, Cardiff West Labour MP Kevin Brennan said he would have preferred a deal with the Liberal Democrats, who were in coalition with Labour from 2000-2003.
But he said the Lib Dems had an "incredible vacuum in leadership" which led them to pull out of talks.
The coalition means the Conservatives will become the main opposition party in the assembly.
Tory group leader Nick Bourne said there were many parts of the Labour-Plaid deal with which he disagreed, but he promised his party would offer "constructive opposition".
The national council has 380 members made up of elected members and representatives from groups and branches within the party.
The deal commits Labour to campaign for a positive vote in a referendum on full law-making powers for the assembly within four years.
Plaid supporters of the coalition have argued that an early referendum would not have been possible if they had chosen to form a "rainbow" coalition with the Conservatives and Liberal Democrats.