The chamber is close to the tomb's terracotta warriors
Historical records describing the tomb of Qin Shihuang, the first emperor of China's Qin dynasty, do not mention the room which is 30 metres (98 feet) deep.
The unopened chamber was found at the site near the old imperial capital of Xian using remote sensing technology.
One expert says it may have been built for the soul of the emperor.
More than 2,000 years old, the chamber is buried inside a pyramidal earth mound 51m (170 feet) high on top of Qin's tomb.
It is situated near the life-size terracotta warriors and has four stair-like walls, says Duan Qingbo, a researcher with the Shaanxi Institute of Archaeology.
The Chinese authorities have not given permission to excavate the site.
It is believed that they wish to perfect archaeological techniques before probing any further, and archaeologists have had to use the sensing technology at the site since 2002.
Despite his brutal methods, Emperor Qin is remembered as a hero in China for forging a unified state.