Emergency services were called to the scene about five miles outside Stornoway, on the Isle of Lewis, just before 10pm last night.
Eleven people, including the driver, were on the bus when the crash happened on the A857 Stornoway to Barvas road.
Four people were injured in total, Police Scotland said.
Three of the four, a women aged 57 and two men, aged 24 and 31, had minor injuries. The injury to the fourth person, believed to be a 17-year-old girl, is not life-threatening.
They were taken to Western Isles Hospital, the Scottish Ambulance Service said.
A spokeswoman said: “There were four patients that were taken (to hospital), three walking wounded and one with a back injury.”