Place the grapes, mint leaves, and sugar into a blender. Blend on high speed for about 10 seconds until the mixture becomes a smooth purée. Add the lime juice and grated ginger, then blend for a few more seconds to fully combine the flavors.

Set a fine-mesh strainer or sieve over a bowl and pour the blended mixture through it. Use a spatula to press down firmly, extracting as much juice as possible. Discard the remaining solids.

Fill two large glasses with ice cubes or frozen grapes. Divide the grape mixture evenly between the glasses, then pour in the seltzer water. Give each glass a gentle stir to combine. Serve immediately while bubbly and fresh.