Cut the watermelon into chunks using a sharp knife and a cutting board, removing any rinds or seeds if present.
Place the watermelon chunks into the blender. Blend on high speed until smooth and completely puréed, about 30-60 seconds.
Pour the blended watermelon through a fine mesh strainer or cheesecloth set over a large bowl to strain out pulp and seeds. Use a spatula or spoon to press the liquid through the strainer for clarity.
Transfer the strained juice back into the blender and add sweetener to taste, if desired. Blend briefly to combine.
Pour the chilled watermelon juice into glasses, garnishing with mint leaves or a small ice cube if desired. Serve immediately for the freshest flavor and vibrant appearance.