The ruins, which are about 36 feet (11 meters) high, were found in the Mexican capital's Tlatelolco area, once a major religious and political center for the Aztec empire. The pyramid is believed to have been built in 1100 or 1200.
Archaeologists also uncovered five skulls and a number of rooms near the pyramid which could date back to 1430s.
The finding signals that the Aztecs, known as the inventors of chocolate, began to develop their civilization earlier than believed.
Mexico's capital is filled with pre-Hispanic ruins. In October 2006, archaeologists found a 15th-century Aztec altar and a stone idol in Tenochtitlan, the capital of the Aztec empire.
The Aztecs, who lived in central Mexico between 14th- 16th centuries AD, were a warlike and deeply religious people, who ruled an empire stretching from the Gulf of Mexico to the Pacific Ocean.