Optional: Rim your margarita glass with salt or Tajín and set aside.
In a cocktail shaker, muddle the jalapeño slices to release heat.
Add tequila, triple sec, watermelon juice, lime juice, and ice.
Shake well until the mixture is chilled.
Strain into a prepared glass over fresh ice.
Garnish with a watermelon wedge and a slice of jalapeño. Enjoy!