After days of heavy rain, part of a mountain tumbled on the bus as it wound through an indigenous area near the mountain village of Zacacoapan, in the Sierra Negra in the central southern state, burying it entirely under mud, fallen trees and boulders.
Mud and rock up to seven metres deep lay on top of the vehicle according to German Garcia, a senior rescue official.
Soldiers worked through the night using diggers to remove the dirt but were only able to reach the body of a woman thought to be in her mid-twenties, according to state officials. No survivors were found.
A few hundred villagers gathered at the site while others prayed around candles in a nearby house.
The federal government said the bus was carrying 60 people, although Garcia estimated about 45 victims were buried.
Felipe Calderon, the Mexican president, sent orders for the interior and defence ministries to join in the effort to dig out the bus, which had left the town of Porfirio Diaz bound for Tehuacan.
Relentless rain in recent days with the start of the annual rainy season has caused flooding in many parts of Mexico.