Add the diced watermelon, tequila, lime juice, simple syrup, and ice to a blender. Blend until the mixture is smooth and well combined.
If you’re going fancy, coat the edge of the glass with salt by rubbing a lime wedge around it and dipping it into a plate filled with salt.
Pour the blended watermelon margarita into the prepared glass.
Garnish with a lime wedge on the rim of the glass for a festive touch, and enjoy!