Begin by cutting fresh watermelon into chunks and blending until smooth in a blender. Strain the mixture through a fine-mesh sieve into a bowl, discarding any pulp. Allow the watermelon juice to sit at room temperature for about 10 minutes.

In a medium mixing bowl, whisk together the glutinous rice flour and sugar until combined. Gradually pour in the room-temperature watermelon juice, stirring continuously to eliminate lumps.

Transfer the mochi batter into a microwave-safe bowl. Cover it loosely with plastic wrap, then microwave on high for about 4 minutes. Stir halfway through cooking if necessary.

Once cooked, carefully remove the bowl from the microwave. Dust your work surface generously with cornstarch. Pour the cooked mochi onto the dusted surface and cool slightly for about 5 minutes. Shape into small balls, using about a tablespoon of mixture for each.

Place the mochi balls on a plate, ensuring they are spread apart. Allow them to cool completely for an additional 15 minutes at room temperature.

Serve chilled or at room temperature for a refreshing treat. Enjoy your Watermelon Mochi!