Sinn Féin endorsing Independent candidate Catherine Connolly in the Presidential Election "gives a clear signal to people that there is a broad, united opposition", party TD Pearse Doherty has said.
He said the party's leadership listened to its members and decided to back Ms Connolly.
Ms Connolly will officially launch her campaign in Dublin this evening.
Speculation had focused on whether the party may back Ms Connolly as part of a wide united left platform that already includes the Social Democrats, Labour, the Green Party, People before Profit-Solidarity and others.
In a statement, Ms Connolly welcomed the party's backing, saying she was "heartened" by their support.
"I believe the coalition of left, centre left parties and independents backing my run for president is a welcome change for Irish politics," she said.
"However if I am to be successful I will be a president for all Irish people, of all political persuasion or none. See you on the road," she added.
Speaking on RTÉ’s Morning Ireland, Mr Doherty said his party has a dual objective of keeping the Government parties "out of the Áras" and having a candidate who will put forward issues such as a united Ireland, neutrality and "who will stand up for marginalised".
"The second priority was also that we want to put this Government out of office, because it is only by having a change of government, that you will actually have a change in the housing situation.
"It is only by getting Fianna Fáil out of government that you will actually be able to put the voices of those with disabilities and those marginalised front and centre with a Sinn Féin-led government.
"So therefore, they're the two priorities that we were looking at and how do we do them. So, that was the decision that we made and Catherine Connolly is an excellent candidate," Mr Doherty said.
"We have now something very unique where the combined opposition is supporting a candidate where Government have two candidates in the race and I think this is going to be a very exciting race and I have no doubt with the support of Sinn Féin that we have energised Catherine Connolly's campaign and our job now is to make sure that she is the next Uachtaráin of Ireland," he added.
Mr Doherty said that Sinn Féin will support Ms Connolly’s campaign financially, the amount of which will be decided in discussions with her team.
"We will fund her campaign. We will support her campaign financially. That's a discussion now that we will sit down with her party campaign. We will be part of that campaign.
"I think the biggest contribution that we're going to bring to Catherine Connolly's campaign is that we have a large organisation that has a reach right across the island of Ireland, that we're embedded in our communities, that we have a large support base that is out there already," he said.
He added that he thinks "people are really, really delighted that we have now a combined opposition".
