Drain mandarin oranges. Pour 1 tablespoon of pineapple juice out of the can and into a large bowl, then drain the crushed pineapple. Discard any additional pineapple juice, or save for another use.
Add sour cream to the reserved 1 tablespoon of pineapple juice. Stir to combine.
Add the mandarin oranges, coconut, marshmallows, and crushed pineapple to the sour cream mixture. Use a spatula to gently fold all of the ingredients together.
Place the salad in the refrigerator for at least one hour, or up to 8 hours, to allow the flavors to blend.
When ready to serve, stir the salad and transfer it to a large bowl. Garnish with additional shredded coconut, if desired. Serve immediately.
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Notes
You can substitute 1 cup of finely chopped fresh pineapple for the canned pineapple. Drain it well before adding it to the salad. 8 large marshmallows is the equivalent of 1 cup of mini marshmallows. If using large marshmallows, snip them into quarters with kitchen shears to create the miniature size. You can substitute ¾ cup of Greek yogurt, ¾ cup of whipped cream, or 1 (8 oz) block of softened cream cheese for the sour cream. Make sure that the cream cheese is at room temperature before using, otherwise the fruit will get smashed while trying to incorporate the cream cheese.