Line a large sheet pan with parchment paper. Place the cubes of watermelon on the baking sheet in a single layer. Make sure the watermelon chunks aren't touching each other.
Place the entire baking sheet in the freezer until the watermelon is frozen, about 2 hours. You can wrap it in plastic wrap, but it isn't necessary.
Transfer the frozen watermelon to an airtight container. You can store your watermelon in freezer bags or in any freezer-safe container. It can last up to 6 months.