The process is to be run from Geneva.
The UN chief noted that the committee is backed by Russia, Turkey and Iran, the main countries involved in the Syrian peace process.
The launch of a process for a new constitution and the formation of the committee started at a conference in Russia in January 2018. Agreeing on the full list of members has taken over 18 months.
Drafting a new constitution is a key element in a broader political process that would include UN-monitored elections to end the eight-year conflict.
The committee is to be made of 150 members, with 50 government delegates, 50 opposition members and the rest chosen by the UN envoy.
A road map for a peace plan, a project led by the UN, was launched in 2012, a year after the start of the Syrian uprising against President Bashar al-Assad.
The Syrian government, backed by Russia and Iran, has largely regained control over most of the country. though rebels, including hardline Islamist factions, still control the northern province of Idlib, on the border with Turkey.
On April 30 the Syrian government, bolstered by Russian air power, began a military offensive against rebels in the provinces of Hama and Idlib.
Since then, government forces have regained dozens of regions in the southern countryside of Idlib and in the northern rural areas of Hama, which were under rebel control.
Kurdish factions and the US control the north-east, having seized that territory from the Islamic State as they defeated the extremist group.
------------------------------------------------------------------------------------