Place the watermelon cubes in a blender container and blend on high speed until puréed. I like to strain mine through a fine-mesh sieve to remove the pulp, but that is optional. Chill the watermelon juice until ready to use.
Salt the rim of 4 glasses by running a lime wedge around the rim and dipping it in coarse salt. Fill with ice.
For each cocktail, combine 3 ounces of watermelon juice, 1½ ounces of tequila, 1-ounce of lime juice, and 1 ounce of triple sec in a cocktail shaker. Fill with ice.
Shake until the outside is frosty, about 15 seconds. Pour into the cocktail glasses. Garnish with a lime wedge and serve immediately.
Notes
For a less pulpy drink: Strain the puréed watermelon through a fine-mesh strainer before chilling it.
Use fresh lime juice. Bottled lime juice doesn’t have the bright, fresh flavor you need for this margarita.
Use frozen watermelon. To make this tequila cocktail any time of year, even when the fruit is out of season, just use frozen watermelon! You can blend it frozen for frozen watermelon margaritas, or thaw the chunks beforehand if you prefer the cocktail on the rocks.
For a sweeter watermelon margarita: With the abundance of fresh watermelon in this recipe for a watermelon margarita, it is usually plenty sweet. But if you taste the watermelon and feel that you need a sweeter drink, add ½ ounce to 1 ounce of honey, agave syrup, or simple syrup to the cocktail shaker with the other ingredients.
To make a frozen watermelon margarita: Either freeze your fresh-cut watermelon for at least 6 hours or use frozen watermelon chunks. Toss the cocktail ingredients (no ice) into a high-speed blender and blend until smooth and thick. You won’t need to serve the frozen version over ice because of the frozen watermelon.
To make spicy watermelon margaritas: If you like your margs with a bit of a kick, swap the salt rim for Tajín, a Mexican chile-lime salt. You can also add a slice or two of jalapeno to the cocktail shaker with the other ingredients for extra heat.