Place the watermelon cubes and pineapple chunks on a clean cutting board, ready for chopping and blending.
Transfer the chopped fruits into a blender jar, ensuring even distribution for a smooth blend.
Blend on high speed until the mixture is thoroughly combined and slightly frothy, about 30-40 seconds. The juice should appear vibrant, with a uniform texture.
Taste and adjust sweetness if desired, then pour the juice into glasses immediately to preserve freshness.
Serve the juice chilled or over ice for a refreshing finish. Optionally, garnish with a small fruit slice or a sprig of mint.