The bus was carrying a group back from an engagement party in Herat province.
Four women and the driver were also killed, a spokesman for the provincial governor told the Reuters news agency.
In a separate development, a suicide car bomber attacked a bakery popular with police in southern Afghanistan, killing five people and injuring 22.
The bakery was close to the police headquarters in Lashkar Gah, the main city in Helmand province.
The UN last month said that 1,462 civilians were killed in the first half of this year, up 15% compared to the same period last year. Insurgents were responsible for about 80% of the killings.
Please can we run out of suicide bombers in the very near future? All they do is dish up pain and suffering on the false promise of paradise.