Foyle MLA Colum Eastwood is challenging Alasdair McDonnell for the leadership of the SDLP.
Mr Eastwood, a former mayor of Derry, was nominated by nine party branches across the North on Friday.
The 32-year-old took the weekend to consider his options and has now said he is accepting the nomination.
“I believe now is the time for a new generation to take forward, not only the party, but the future direction of the country,” he said.
Mr Eastwood pledged to pursue “a new brand of progressive nationalism - one that is not defined by birth but based on radical and realistic proposals and policies for a better future”.
“Those of us who want to achieve a united Ireland in the future must give our all to make Northern Ireland work today,” he added.
“As a proud nationalist, I believe deeply in a united Ireland. I want progressive nationalism to go much further on unity than the mere sloganising that until now has been trying to pass itself off as strategy.
“I want the SDLP to set out exactly what a new united Ireland will look like, and to set about convincing all the people of Ireland of the value of our convictions.”
Mr Eastwood, a former councillor and an MLA since 2011, explained he was “hungry for change and ready to lead”.
The election for leader is due to take place at the SDLP party conference in November.
SDLP deputy leader Dolores Kelly is expected to face a challenge for the role from its health spokesman Fearghal McKinney MLA.
Mrs Kelly said it was difficult for Alasdair McDonnell to lead the Assembly team from Westminster and believed Mr Eastwood is the right person to lead the party.
“I think it’s time for a new generation,” she added, suggesting he would appeal to a new generation of voters.
She also said her experience would complement Mr Eastwood and move the party forward.
Alasdair McDonnell, a South Belfast Westminster MP, has lead the SDLP for nearly four years.
Often described as a ‘political bruiser’, he won the contest to replace Margaret Richie as SDLP leader in November 2011.