Gather all ingredients, and preheat the oven to 350 degrees F (175 degrees C).

Place sweet potatoes in a large saucepan with enough water to cover. Bring to a boil and cook until tender, about 15 minutes. Remove from heat, drain, and mash.

Place mashed sweet potatoes in a large bowl. Add brown sugar, butter, orange juice, and cinnamon; mix with an electric mixer until blended.

Spread evenly into a 9×13-inch baking dish. Sprinkle marshmallows over top.

Bake in the preheated oven until casserole is heated through and marshmallows are puffed and golden brown, 25 to 30 minutes.

Serve and enjoy!