The bus was traveling from the city of Quetta to Karachi and had nearly 48 people on board, local media reported, citing an official.
"Due to speeding, the coach crashed into the pillar of a bridge while taking a U-turn near Lasbela [district]. The vehicle subsequently careened into a ravine and then caught fire," Lasbela Assistant Commissioner Hamza Anjum was quoted as saying by the media.
At least 41 people were killed in the incident and three others, including a woman and a child, were rescued from the wreckage alive, the district official said. However, one of the survivors succumbed to injuries on the way to the hospital.
The authorities will establish the identities of those killed in the crash by DNA testing, as the bodies are badly mutilated, Anjum added.
The search and rescue operations continue.