The attack happened in Golweyn village, controlled by Al Qaeda-linked Al Shabab, 110 kilometres to southwest of the capital, Mogadishu.
A roadside bomb killed at least 20 people in a minibus in Somalia's Lower Shabelle region on Thursday evening, the region's governor said.
There was no immediate claim of responsibility, but the governor blamed the Al Qaida-linked Al Shabab militant group.
Most of the victims were women, Ibrahim Aden Ali said. He called the blast a "national disaster."
Regional security official Nur Abdullahi said six other people were injured.
The massive bomb buried beside the road struck the vehicle near Golweyn village and largely destroyed it, Abdullahi said.
Isaq Haji, a nurse at a hospital in Marka town 90 kilometres (55 miles) south of the capital, Mogadishu, said they had received four victims with "horrific" wounds.
The Somalia-based Al Shabab claims control over parts of the largely coastal Lower Shabelle region, which has been a focus of efforts to counter the group by a 22,000-strong multinational African Union force.
The area where the bomb exploded is a contested one. Roadside bombs are commonly used by Al Shabab.
Civilians often have been casualties in this long-chaotic Horn of Africa country. Now hundreds of thousands of Somalis are on the move as a drought threatens roughly half of the country's population of 12 million.
New Somali-American President Mohamed Abdullahi Mohamed, who was elected in February, has promised to make security a priority as the weak central government tries to assert itself beyond the capital and some other urban areas.