Plaid Cymru members have voted overwhelmingly in favour of a co-operation agreement with Welsh Labour.
The motion passed with a majority of 94% after a vote in the Senedd on Saturday 27 November.
Plaid Cymru Leader Adam Price called it a “huge step forward” for Wales and its democracy. The party leader had previously called the pact a "down-payment on independence".
The new deal was already endorsed by Plaid Cymru and Labour executive committees last week, but has now officially been ratified after passing the final hurdle.
It will come into effect upon signing by the First Minister and the Leader of Plaid Cymru.
The parties will begin to work together on a shared policy programme covering 46 areas, including free school meals for all primary school children, a commitment to tackle the second homes crisis and long-term reform of the Senedd.
Speaking after the vote Plaid Cymru Leader Adam Price MS said: “This is a huge step forward for Wales and our democracy. The Co-Operation Agreement will bring immediate, tangible and long-term benefit for the people of Wales.
“All primary school children will now receive free school meals; there will be free childcare for all two-year-olds; and radical action to tackle the housing crisis.
“There will be stability payments to support family farms; exploration of an accelerated pathway to net zero by 2035; the creation of Ynni Cymru - a company to expand community-owned renewable energy generation; and a new and reformed Senedd – bigger, more diverse, and gender balanced in law.”
The Plaid leader hopes the new agreement will change the lives of thousands of people across the whole of Wales.
He added: “Almost a quarter of a century ago, people in Wales voted for self-government for Wales, with a promise of a new type of politics. They placed their trust in a new democracy with an instruction to work differently – inclusively and co-operatively.
“I am therefore pleased this pioneering Co-operation Agreement is founded on common ground on a range of issues that will make a long-lasting difference to people’s lives. And I cannot wait to get to work, for the people of Wales.”
The Conservatives have strongly criticised the agreement, saying it will inflict "despair" on Wales and cause "constitutional chaos."
Meanwhile, the Liberal Democrats have said they're "concerned" that the agreement does not include measures to tackle the "crisis" in healthcare provision in Wales.