Cut watermelon into large cubes (discarding rind) and freeze 6 hours or overnight.
Place watermelon chunks in a high-speed blender. Top with tequila, Triple Sec, and lime juice.
Blend until thick and smooth. Pour into two margarita glasses and serve immediately.
Optional: Line the glass rims with salt and garnish with lime and watermelon wedges.
Notes
This recipe can easily be doubled or tripled to make a large batch for parties.
You can use a Blanco or Anejo tequila. Any brand will do, but a higher quality one will have better flavor.
In a pinch, you can just add ice to the fresh watermelon. But try to plan ahead for the best consistency and flavor. Otherwise, it will have a watered-down flavor.
If too thick: Add a bit more alcohol or water to dilute.
If too thin: Blend in a bit more of the watermelon pieces.
The margarita will separate a bit while it sits. Simply stir again before serving.