Sellstrom, who was given the task by U.N. Secretary-General Ban Ki-moon, is a project manager at a Swedish research institute who also has taught at universities in the United States and served as director at the Swedish Defense and Security Research Institute.
U.N. spokesman Martin Nesirky described Sellstrom to reporters in New York as "an accomplished scientist with a solid background in disarmament and international security."
Sellstrom served in various capacities with the United Nations, including as a senior adviser on the disarmament of Iraq, a U.N. release stated.
Ban announced the investigation last week following allegations of a chemical-weapons attack in Khan al-Assal, southwest of Aleppo, that killed at least 19 civilians and injured 69 others. Syrian rebels alleged there was a second attack in suburban Damascus.
The rebels and the regime of President Bashar Assad blamed each other for the alleged use of chemical weapons.
France and Britain also requested the United Nations investigate the allegations.
"The terms of reference for the mission are being finalized in consultation with the Organization for the Prohibition of Chemical Weapons and the World Health Organization," Nesirky said.
"While the terms of reference are being finalized, work is already well under way so that the mission can be dispatched quickly."