De Mistura is expected to meet senior representatives from the United States, Britain, Saudi Arabia, Jordan, Germany, France and Egypt on September 14.
The Geneva talks would come two days after a scheduled meeting between the UN envoy and representatives from Russia, Turkey and Iran on September 11-12.
"This is going to be an opportunity to discuss the way ahead on the political process," UN spokeswoman Alessandra Vellucci said on Tuesday, Presstv reported.
De Mistura is tasked with picking members of Syria's Constitutional Committee after receiving nominations from the Syrian government and the opposition.
The UN spokeswoman said on Friday that de Mistura had invited Iran, Russia and Turkey to talks due to be held in Geneva on forming Syria's Constitutional Committee.
"Special Envoy de Mistura has invited Iran, the Russian Federation and Turkey for formal consultations at the United Nations office at Geneva on 11 and 12th September 2018 to work on the Constitutional Committee," Vellucci said at a news conference.
Iran, Russia and Turkey, as guarantor states for a peace process in Syria, expressed their firm determination in late July to counter any plot aimed at partitioning the Arab country.
Turkish state television reported on Monday that the presidents of Iran, Russia, and Turkey will hold their third round of tripartite summit in Iran next week.
Iranian President Hassan Rouhani will host his Turkish and Russian counterparts Recep Tayyip Erdogan and Vladimir Putin, respectively, on September 7, state-run TRT Haber television said.
In a meeting with Syrian President Bashar al-Assad in Damascus on Sunday, Iran's Defense Minister Brigadier General Amir Hatami hailed the steadfastness of the Syrian people and government forces in their battle against terrorism, expressing confidence that they will achieve a final victory in this path.
"Not only regional people, but people around the world are indebted to the fight carried out against terrorists in Syria," Hatami said.
Syria has been gripped by foreign-backed militancy since March 2011. The Syrian government says the Israeli regime and its Western and regional allies are aiding Takfiri terrorist groups wreaking havoc in the country.
All initiatives with the purpose of putting an end to Syria's seven-year-old conflict have failed so far.