Staffan de Mistura, a veteran diplomat and former UN special envoy to Afghanistan, will replace Lakhdar Brahimi as the UN mediator for Syria, diplomatic sources said on Wednesday.
UN diplomats in New York said UN Secretary General Ban Ki-moon would announce the choice of de Mistura, a dual citizen of Italy and Sweden, later on Wednesday.
Many diplomats have said prospects for a negotiated settlement of the war have dimmed amid worsening violence.
Brahimi stepped down on May 31, frustrated by international deadlock over how to end the three year civil war in Syria.
He had long threatened to quit, just as his predecessor - former UN Secretary-General Kofi Annan - did in 2012.
Annan resigned after six months as the UN and Arab League joint special representative on Syria, accusing the UN Security Council of failing to unite behind efforts to end the fighting.
Russia, supported by China, has vetoed four UN Security Council resolutions threatening action against its ally, Syrian President Bashar al-Assad.
The United Nations says more than 150,000 people have been killed, some 10.8 million people in Syria need help, of which 4.7 million are in hard-to-reach areas, while another three million have fled the conflict.
De Mistura, has worked with the United Nations in Somalia, the Middle East, the Balkans, Nepal, Iraq and Afghanistan during the past three decades. He was most recently the UN special envoy in Afghanistan in 2010-2011.
It was not immediately clear if he would be a joint UN and Arab League envoy. Ban Ki-moon met Nabil el-Araby, head of the Cairo-based Arab League, in June to discuss whether a new appointee would still be a joint appointee.