Experts at the National Institute of Anthropology and History said Thursday the bar's metallic content matches the gold, silver and copper mix that characterized Aztec gold artifacts found since. It also matched measurements given by the conquerors.
The gold bar was found in 1981 in a former canal area just west of the old Aztec capital, then known as Tenochtitlan.
It was located roughly on the route taken by the Spaniards when they fled an uprising on June 30, 1520.
Grudgingly welcomed into the city by Emperor Moctezuma, the forces of Hernán Cortés quickly turned Moctezuma into a virtual prisoner.
They forced the Aztecs to melt down treasures to form gold bars, but when city residents rose up against them, the Spaniards were forced to flee. abandoning many of their men and much of the treasure.
The Spaniards, aided by indigenous allies, returned in 1521 to complete their conquest of the city.