Begun about 2,000 years ago to keep out marauding neighbours and renovated by various imperial dynasties, the Wall was under construction for centuries and is China's most famous landmark.
And they have said that the Wall may extend even further.
The new section of the Wall, discovered during a field survey in 1988, makes its total length to 7,200km (4,470 miles), the official Xinhua news agency reported.
Begun about 2,000 years ago to keep out marauding neighbours and renovated by various imperial dynasties, the Wall was under construction for centuries and is China's most famous landmark.
There has been no independent verification of the report.
The province of Xinjiang - home to Muslim Uighurs - has seen strong separatist unrest, with a campaign of bombings in recent years.
Silk Road
In the report by Xinhua, experts were quoted as saying that the Wall was built to protect merchants travelling on the ancient Silk Road and runs parallel to the famed Silk Road.
Previously, the Wall's westernmost end was thought to be a remote fortress at the Jiayu Pass in Gansu.
"There is no doubt this is part of the Great Wall as it consists of the city wall and beacon towers, forming a complete defence system," Luo Zhewen, president of the China Society of Cultural Heritage, told Xinhua.
The experts said the shape and size of the new section resembles other sections of the Wall, with its yellow sandy stone and jarrah branches.
A large number of arrowheads have been found near the new site, indicating battles took place nearby, Xinhua reported.
The oldest part of the Wall was completed by the Qin dynasty, after it united China in 221 BC. It runs further north across the wild mountains that form the southern edge of the Mongolian steppe.