They say it could be 2,700 years old, making it the oldest burial site in a pyramid in Mesoamerica.
Inside the tomb, they found the skeletons of four people, one of them of a man surrounded by jade and amber.
The researchers believe he could have been a high priest or ruler of Chiapa de Corzo, a prominent settlement at the time.
The team of archaeologists from Brigham Young University in the US, the Mexican National Institute of History and Anthropology, and the National Autonomous University of Mexico said the discovery suggests that the use of pyramids as burial sites goes back much further than previously thought and could pre-date Mayan culture.
“A thousand years before we see pyramid tombs used for royal burials in the Mayan region, here in Chiapa de Corzo, they were already burying members of the elite inside pyramids. We’re talking about 700BC,” the team explained.
The tomb is located inside a pyramid which would have been around 6m-7m (20ft -22ft) high when it was first built, with mud stairs leading to a temple on top of the structure.
The researchers found the tomb deep inside the pyramid. After 24 hours of digging, they unearthed a funerary chamber measuring 4m by 3m. The chamber held the bones of three people: a middle-aged man, a child of about one, and a young man.
The middle-aged man was richly adorned, his mouth was covered with a shell and his teeth were incrusted with jade. He also wore bracelets, anklets, necklaces and what the archaeologists believe to be a funerary mask with eyes made of green obsidian.
Investigators from the Archaeological Project Chiapa de Corzo say that judging by the wealth of jewellery he was buried with, he would have been of high rank.
They said the two other bodies may have been added to the tomb to accompany the dead man to the afterworld and were possibly sacrificed.
The researchers say the position of the bones suggests the baby was carefully placed in the tomb, while the young man was possibly thrown into the burial chamber.
In an annex to the main chamber, the archaeologists found another smaller room containing the skeleton of a woman, also richly adorned with amber and pendants depicting birds and a monkey.
The number and variety of the offerings suggests the people living in this region at the time were trading with places as far away as the Gulf coast of Mexico and Valle Motagua in Guatemala, which was rich in jade.
Bruce Bachand, Emiliano Gallaga and Lynneth Lowe of the archaeological team say the discovery suggests humans have been living in Chiapas at least since 1200BC.