The tomb held a man aged around 50, who was buried with jade collars, pyrite and obsidian artefacts and ceramic vessels.
Archaeologist Emiliano Gallaga said the tomb dates to between 500 and 700BC.
Based on the layers in which it was found and the tomb’s unusual wooden construction, Mr Gallaga said: “We think this is one of the earliest discoveries of the use of a pyramid as a tomb, not only as a religious site or temple.”
Pre-Hispanic cultures built pyramids mainly as representations of the levels leading from the underworld to the sky; the highest point usually held a temple.
The tomb was found at a site built by Zoque Indians in Chiapa de Corzo, in southern Chiapas state.
It may be almost 1,000 years older than the better-known pyramid tomb of the Mayan ruler Pakal at the Palenque archaeological site, also in Chiapas.
The man – probably a high priest or ruler of Chiapa de Corzo, a prominent settlement at the time – was buried in a stone chamber.
Marks in the wall indicate wooden roof supports were used to create the tomb, but the wood long ago collapsed under the weight of the pyramid built above.