The frieze, which is eight metres long and two metres wide (26ft by six feet), shows three figures decorated with quetzal feathers and jade sitting atop the head of a mountain spirit.
It was found at the pre-Columbian archaeological site of Holmul.
Site director Francisco Estrada-Belli called it it a once-in-a lifetime find.
Snake Lords v Tikal
The frieze was found below a 20m-high (65ft) pyramid which was built over it in the 8th Century.
“The preservation is wonderful because it was very carefully packed with dirt before they started building over it,” Mr Estrada-Belli said.
The sculpture is believed to depict the crowning of a new Mayan leader in about AD590.
It also bears an inscription made up of 30 glyphs, which was deciphered by Harvard University expert Alex Tokovinine.
The inscription says that the carving was commissioned by the ruler of a nearby city-state, Ajwosaj ChanK’inich.
The archaeologists say the frieze and its inscription shed light on a classical period of Maya rule in which two rival kingdoms, Tikal and the Snake Lords, fought for control of the region.
Mr Tokovinine says the inscription suggests that Ajwosaj, who was a vassal of the Snake Lords, came to the site to re-establish the local political and religious order after Holmul, which had supported the Tikal kingdom, had switched sides.