Place sweet potatoes in an 8x8 inch square baking dish. Pour caramel over potatoes. Sprinkle with sugar and dot with butter.
Bake in preheated oven 30 minutes, until potatoes are tender and mixture is bubbly. Remove from oven and sprinkle marshmallows evenly over potatoes. Return to oven until marshmallows are melted and browned, 5 to 10 minutes more.