Rim a Collins glass with salt. Fill the glass with ice and set aside.
Add tequila, grapefruit juice, lime juice, and simple syrup to a cocktail shaker.
Fill the shaker with ice and shake for 10-15 seconds until well chilled.
Strain the mixture into the prepared Collins glass.
Top with sparkling soda water and stir gently with a bar spoon.
Garnish with a grapefruit wedge and serve immediately.
Notes
Fresh-squeezed grapefruit juice works best for this recipe. Store-bought juice can be used in a pinch.The cocktail can be made in large batches for parties. Mix all ingredients except soda water in advance and add the bubbles just before serving.Keep ingredients cold for the best results. Warm citrus juice can make the drink taste flat.The salt rim is optional but adds a nice contrast to the sweet and tart flavors.