The segment was found in the north-western region of Ningxia which was crucial to imperial China's border defences, the state news agency Xinhua reports.
It stretches some 80 kilometres (50 miles) along the southern slope of Helan Mountain, about 40 km (25 m) from the Ningxia provincial capital, Yinchuan.
Other sections of the vast wall - a Chinese and world heritage monument - have been uncovered in the past in remote parts of the countryside.
The newly discovered section was built in 1531 in a meandering line with three watchtowers added in 1540.
The Great Wall of China
Built from the 7th century BC onwards to defend the northern borders
6,000 km long and, on average, 3.5m high and 4.5 m wide
But the sands which wash across the arid surrounding countryside gradually buried it.
The segment discovered is seven metres (23 feet) high and 6.5 metres (22 feet) wide at its base.
One watchtower remains in good condition, Xinhua says.
Other sites
Parts of the Wall were being built in Ningxia as far back as the Warring States period of 475-221 BC.
Many people associate the Wall with the sections north of Beijing, which are a popular tourist destination.
But it stretches about 6,000 km (3,700 miles) through the north, from Gansu Province in the west to Shanhaiguan Pass on the shores of Bohai Bay in the east.
In August, Chinese experts announced the discovery of other important ruins in Gansu, which borders Ningxia.
Thirty unrecorded Great Wall beacon towers, two fortified castles and two auxiliary buildings were discovered during their expedition.
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