SDLP votes in favour of partnership with Fianna Fail
121 members voted in favour of the partnership, with 53 against.
The SDLP membership has voted in favour of forming a partnership with Fianna Fail.
The motion passed by 121 votes to 53 at a special party conference in Newry on Saturday.
Nearly 70% of members voted in favour of the partnership with Fianna Fail, while the other 53 voted for an alternative proposal of a partnership with Fianna Fail, Fine Gael and Labour.
The partnership had been opposed by some high profile members of the party including South Belfast MLA Claire Hanna and Councillor Mairia Cahill.
It was also opposed by the party's youth and LGBT wings.
Speaking after the vote Ms Hanna said "I came believing an exclusive arrangement was not a good idea, I have not changed my mind".
Following the 1998 Good Friday Agreement the SDLP were the largest nationalist party in Northern Ireland, but their electoral success has declined in recent years with the rise in popularity of Sinn Fein.
Fianna Fail are currently the largest opposition party to Fine Gael in the Republic of Ireland, but have spent more years in government than any other party.
Following the vote the SDLP leadership issued a statement welcoming the decision to agree to the partnership.
The statement acknowledged that the choice had not been easy, but noted that "the SDLP has a strong history of making tough decisions in the interests of the people we represent."
By making the decision the leadership said that the party were putting people first.
"We understand that the political environment across these islands is changing rapidly. Parties, too, must be prepared to embrace change and respond to the challenges we all face. Business as usual is no longer an option," the statement read
"Together, SDLP and Fianna Fail members, representatives and activists will forge a new relationship based on our proud and shared tradition of democratic struggle on this island.
"We will work together to begin in an unprecedented programme of public engagement in Northern Ireland, framing an ambitious and practical policy agenda for the future of this island and our people.
The statement acknowledged that the party had not unanimously backed the move, and said the leadership intended to reach out to those unhappy with the decision in the coming weeks.
"This partnership will be built on a genuine desire to deliver for people across Ireland, driven by an ambition to see power returned to local politicians and tasked with changing the lives of people who have for too long been let down by our politics here," the statement said.
"For some members this decision will not have been their desired outcome. The leadership recognise and are sensitive to that fact. We will be reaching out to all strands within our party in the coming weeks and months to ensure we move forward together."
Announcing the plan at an event last month Mr Eastwood said the two parties had agreed to "work in partnership on an unprecedented programme of public engagement in Northern Ireland" organised around three central themes.
The first would be "a politics that works", to address the failures of current political practice here and to explore the need for a new economic model.
The second would be "better public services".
The third, "uniting Ireland's people", would involve addressing "practical steps for greater co-operation and the shape of future possible constitutional arrangements should a unification poll be held and passed".
Belfast Telegraph Digital