The bus plunged through a guardrail and rolled down an embankment in the northern state of San Luis Potosi on Wednesday.
Local media reported that the bus departed from an area near Mexico City and was heading towards the US border. Photos from the scene suggest the bus flipped onto its side.
Immigrant smugglers frequently pile migrants onto cargo trucks and buses to cross Mexico en route to the United States.
It was the latest in a series of fatal accidents and migrant drownings over the past week.
Immigrant activists say a crackdown on migrant crossing has led some to take riskier routes to the US border.
“More and more people are choosing to take more dangerous roads,” wrote immigrant activist Irineo Mujica. “The number of people dying trying to cross Mexico is increasing.”
On Tuesday, authorities in the Gulf Coast state of Veracruz said six migrants drowned off Mexico’s Gulf Coast and one remains missing. Apparently all were from Honduras. Smugglers sometimes take migrants in open boats across the Gulf to avoid immigration checkpoints on the highways.
Four migrants, also Hondurans, were pulled alive from the ocean by rescuers after their 25ft undecked boat apparently capsized offshore.
In a separate incident, immigration officials said this week that a migrant father and his 7-year-old son were found dead in the Suchiate River, which marks the border between Mexico and Guatemala.
Mexico’s national immigration institute said the 36-year-old and his son were from El Salvador.
Migrants frequently wade, swim or take rafts and boats across the border river to Mexico in their attempts to reach the US border.
Mexican authorities have stepped up border inspections along the Guatemalan border, but tens of thousands of Central Americans are still crossing, often with the help of smugglers.
On Saturday, four migrants died and at least 16 were injured in a traffic accident in a town not far from the Guatemalan border. The migrants were being transported on the back of a cargo truck in the township of San Juan Chamula, just north of the tourist town of San Cristobal de las Casas.
The prosecutor’s office did not identify the nationalities of the dead men, but said the injured included nine people from Honduras, four from Guatemala, one Nicaraguan and one Ecuadorian, as well as one person whose nationality was not Claire.
A much larger truck carrying migrants overturned on a highway near the town of Tuxtla Gutierrez in December, killing at least 56 people.