Heat oven to 350°F. Spray 12x8-inch (2-quart) glass baking dish with nonstick cooking spray.
Place sweet potatoes in sprayed baking dish. Pour margarine over potatoes. Sprinkle with brown sugar and salt. Top with marshmallows.
Bake at 350°F. for 25 to 30 minutes or until potatoes are thoroughly heated and marshmallows are lightly browned.