Mexican archeologists found the ruins, which are about 36 feet high, in the central Tlatelolco area, once a major religious and political centre for the Aztec elite.
Since the discovery of another pyramid at the site 15 years ago, historians have thought Tlatelolco was founded by the Aztecs in 1325, the same year as the twin city of Tenochtitlan nearby, the capital of the Aztec empire, which the Spanish razed in 1521 to found Mexico City, conquering the Aztecs.
The pyramid, found last month could have been built in 1100 or 1200, signalling the Aztecs began to develop their civilization in the mountains of central Mexico earlier than believed.
“We have found the stairs of this, much older pyramid. The (Aztec) timeline is going to need to be revised,” archaeologist Patricia Ledesma said.
Tlatelolco, visited by thousands of tourists for its pre-Hispanic ruins and colonial-era Spanish church and convent, is also infamous for the 1968 massacre of leftist students by state security forces there, days before Mexico hosted the Olympic Games.
The dig has also turned up a sculpture that could be of the Aztec rain god Tlaloc, or of the god of the sky and earth Tezcatlipoca, five skulls and a series of rooms near the pyramid that could date from 1431.