He is to replace Lakhdar Brahimi who resigned at the end of May after two rounds of peace talks yielded no concrete results. Syrian President Bashar al-Assad was then re-elected in June.
His nomination was confirmed to members of the UN Security Council but has not yet been announced officially by the United Nations.
One diplomat said de Mistura would represent the United Nations and "have an Arab deputy."
Brahimi, a well-respected and seasoned Algerian diplomat with extensive mediation experience, was joint representative of both the United Nations and Arab League to Syria.
De Mistura, 67, was born in Sweden and holds Italian- Swedish nationality. He is a former deputy Italian foreign minister and has worked for the United Nations for more than three decades.
He was most notably UN special representative to Iraq from 2007-2009 and special representative to Afghanistan from 2010-2011.
Brahimi, who served as the envoy to Syria from August 2012 until May 2014, used all of his diplomatic skills to coax Assad and Syria's fractious opposition to the negotiating table in Geneva earlier this year.
The United Nations says the number of Syrians in urgent need of humanitarian aid has jumped to 10.8 million - nearly half of Syria's population of 22 million.
