Deputy provincial police chief Col. Asadullah Ensafi said the blast occurred in the Andar district as the bus travelled from one village to another just before dusk.
He said the dead include 14 women, three men and a child. Ensafi said the blast wounded five women and two were in critical condition.
Ensafi said he had no other details, as the remote area was not easily accessible to security forces.
Andar is one of the few districts in Ghazni where the Taliban retain some measure of control and often attack security forces, mostly by laying bombs along roads.
Roadside bombs are the Taliban's weapon of choice and are responsible for the overwhelming majority of civilian casualties.
In a statement Sunday night, the NATO-led International Security Assistance Force condemned the attack, saying it would "fight against extremists whose senseless acts endanger innocent women and children."
The U.N. said in the first six months of this year, 1,319 civilians were killed and 2,533 were wounded in the ongoing 12-year Afghan conflict, the majority of them by roadside bombs.
Earlier Sunday, a bomb apparently targeting a group of soldiers killed a civilian in a market in the capital, Kabul.
Afghan Defense Ministry spokesman Gen.
A man who identified himself as Ziaudin said his 10-year-old daughter was killed. A witness, Hashmatullah, said four civilians were wounded in addition to the soldiers. He said the bomb was placed under vegetables in a shop.
Like many Afghans, the two men only use one name.
There has been a spike in violence around Afghanistan in recent months as the insurgents try to take advantage of a security handover from foreign forces to the Afghans. The handover is the latest step in the gradual withdrawal of troops from the U.S.-led international military coalition, which will be completed at the end of 2014.
Associated Press writer Patrick Quinn contributed to this report.