Mohibullah Mohib, spokesman for the police chief in Farah province, said 15 others were wounded with most in critical condition, indicating the death toll could rise.
The bus was traveling on a main highway between the western city of Herat and the southern city of Kandahar, AP reported.
No one immediately claimed responsibility, but Taliban insurgents operate in the region and frequently use roadside bombs to target government officials and security forces.
The Taliban have kept up a steady tempo of attacks even as they have held several rounds of peace talks with the United States aimed at ending the 18-year war.
The attack came a day after the UN mission in Afghanistan released a report saying that most civilian deaths in the first half of the year were caused by Afghan and NATO forces. The report apparently referred to civilians killed during Afghan and US military operations against insurgents.
The UN report said 403 civilians were killed by Afghan forces in the first six months of the year and another 314 by international forces, a total of 717. That's compared to 531 killed by the Taliban, a Daesh (ISIL or ISIS) affiliate and other militants during the same period. It said 300 of those killed by militants were directly targeted.
The UN said the leading cause of civilian deaths and injuries was "ground engagements," which caused one in three casualties. Roadside bombs were a close second, accounting for 28%. Afghanistan is one of the most heavily mined countries in the world — a legacy of decades of war.
The Taliban, who effectively control half the country, have been meeting with US envoy Zalmay Khalilzad since late last year.
They appear to be closing in on an agreement whereby American forces would withdraw from Afghanistan in return for guarantees that it would not be used as a launch-pad for international terror attacks.