In a medium bowl, mix cranberries and sugar. In the refrigerator, allow the mixture to stand 4 hours.
Place cranberry and sugar mixture in a colander. Place colander in a large bowl. Allow the mixture to drain 8 hours, or overnight.
Discard drained juices. Place cranberry mixture in a medium bowl. Stir in grapes, walnuts, marshmallows and thawed frozen whipped topping. Chill in the refrigerator until serving.
