Place your watermelon juice, lime juice, tequila, triple sec, and simple syrup into a shaker with ice. Shake until the drink is chilled.
Add a few tablespoons of tajin to a small plate. Run a lime wedge around your glasses and dip it into the tajin.
Pour the margarita into your glasses and top it with a lime slice.
To Store: I recommend making only what you will drink that day. If you have leftovers, you can store the watermelon margaritas in a pitcher in the fridge for a few hours to a day.
Add a few jalapeño slices to your cocktail shaker to make this a spicy watermelon margarita.
If your watermelon is fully ripe and sweet, you may not want to add a sweetener.
I love using fresh watermelon for this recipe, but I have made it with the bottled watermelon juice when I want a quicker and easier way to prepare these. You can find watermelon juice near the orange juice in the refrigerated section. I like the simple brand of watermelon juice.
To keep your margarita cool and refreshing, pour it over ice. You can also chill your cocktail glasses in the fridge or freezer for about 15 minutes before serving.
This recipe makes enough for 2 margaritas, but feel free to double or triple it for a crowd.