As well as being charged with supporting the country's political process and ensuring the protection of its civilians, Leila Zerrougui of Algeria will offer to use her office to mediate the "full implementation of the political agreement of December 31, 2016", according to a statement obtained by AFP.
The agreement sets out the general principles for holding elections and the "peaceful transfer of power and the consolidation of stability in DRC," the statement said.
President Joseph Kabila, who has been in power since 2001, refused to step down at end of his two-term limit in December 2016, stoking a bloody spiral of violence.
Under an agreement brokered by the Catholic Church, he was allowed to stay in office provided new elections were held in 2017, but they have since been pushed back to December 23, 2018.
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In January Kabila claimed the UN's mission in the country, known as Monusco, had "eradicated" no armed group in nearly 20 years.
He also warned the mission not to consider the country "under the care of the United Nations".
Also in January, UN Secretary General Antonio Guterres reaffirmed the UN's willingness to continue to address the country's security challenges, despite the tensions with the government.
Zerrougui served as the deputy head of Monusco from 2008 to 2012 and takes over as United Nations special representative and head of Monusco from Maman Sidikou.
Monusco is the largest UN mission and has been present in the country since 1999.
Sprawling, mineral-rich but mired in poverty, DRC is in the grip of overlapping political and ethnic crises, and much of the country's east is in the hands of rival militia groups competing over resources.