The bus was carrying 14 elderly members of the First Baptist Church of New Braunfels, Texas.
The cause of Wednesday's crash was not immediately known. The accident happened at 12:30 local time (17:30 GMT) as the church members were returning from a camp retreat.
Texas Governor Greg Abbot expressed "deepest condolences to the victims and the families of those involved in today's tragic event".
Only the driver of the pick-up truck and one of the minibus passengers survived, the latter in a critical condition.