Rub a lime wedge around the edge of a glass, then dip the rim of the glass in salt and fill the glass with ice.

Add the watermelon juice, lime juice, simple syrup, and tequila to a cocktail shaker with ice. Secure the shaker with a lid, then shake for 30 seconds to a minute, or until the drink is very cold.

Pop the lid off the shaker and pour the drink into your prepared glass. Then, fill the glass the rest of the way with grapefruit soda.