A provincial official says the bomb tore through the bus, which was carrying mostly women and children. The injured were taken to Herat Regional Hospital for treatment.
No group has claimed responsibility. Taliban insurgents, however, operate in the region and frequently use roadside bombs to target government officials and security forces, even as peace talks involving U.S. officials and Taliban representatives are scheduled to resume.
The two sides hope to establish a timetable for the withdrawal of foreign forces in exchange for security guarantees by the Taliban.
The deadly violence Wednesday came a day after the United Nations reported that nearly 4,000 Afghan civilians were killed or wounded in the first half of 2019.
The U.N. Afghan mission noted in its report released Tuesday that more civilians were killed by government and NATO-led troops than by the Taliban and other insurgent groups in the first half of 2019.