Dip the rim of a margarita glass in lime juice then margarita salt. Hold the glass upside down for 15 seconds for the salt to set. Fill the glass with ice.
Place 3 or 4 wedges of watermelon, rind removed, into a blender and blend for 15 seconds. Strain watermelon puree into a measuring cup.
Mix ⅓ cup of the watermelon puree with the tequila, triple sec and lime juice in a cocktail shaker over ice. Strain into the ice-filled glass.