Place the watermelon cubes, tequila, lime juice, simple syrup, and ice in a blender. Blend until smooth and fully combined.
If desired, rim the glass with salt by rubbing a lime wedge around the edge and dipping it in a plate of salt.
Pour the blended watermelon margarita mixture into the prepared glass.
Add a lime wedge to the rim of the glass for a festive touch, and enjoy!