Add the watermelon cubes to a blender and blend until smooth.
Strain the juice through a fine-mesh sieve into a pitcher to remove pulp.
Fill a cocktail shaker with ice and add 4 ounces of watermelon juice, tequila, orange liqueur, lime juice, simple syrup, and the jalapeño slice.
Shake vigorously for about 20 seconds.
Strain into a glass filled with fresh ice.
Garnish with a watermelon wedge and a jalapeño slice.