Use a kitchen knife to carefully cut the watermelon in half, then peel off the thick green rind with your fingers or a knife, exposing the bright red flesh. Cut the flesh into smaller chunks, approximately 1-inch in size, on a cutting board.
Transfer the watermelon chunks into the blender jar, ensuring all the pieces are evenly spread for smooth blending.
Secure the lid on the blender and blend on high speed for 30-45 seconds until the mixture appears uniformly smooth and slightly frothy. You should see a vibrant, deep red liquid with no large chunks remaining.
Place a fine-mesh strainer or sieve over a large bowl or pitcher, and pour the blended watermelon through it. Use a spoon or rubber spatula to press the pulp gently, extracting as much juice as possible and leaving behind seed and fiber remnants.
Pour the strained watermelon juice into glasses. Optionally, chill in the refrigerator for 10-15 minutes before serving for a more refreshing experience.
Serve the bright pink juice immediately, garnished with a small sprig of mint or a thin slice of watermelon if desired.
