Place the watermelon cubes in the blender pitcher.
Add the lime juice and mint leaves to the blender.
Blend on high speed until the mixture is smooth and uniform, about 30 seconds. The liquid should be vibrant pink and free of chunks.
Pour the blended mixture evenly into popsicle molds, leaving about 0.5 inch space at the top. Insert popsicle sticks if not built-in.
Place the molds in the freezer and freeze for at least 4 hours, or until completely solid.
Once frozen, run the molds under warm water for a few seconds to loosen, then carefully remove the popsicles. Garnish with mint leaves if desired and serve immediately.