Place the sugar, 1/4 cup water and 2 sprigs rosemary in a saucepan and bring to a simmer over low heat to dissolve the sugar. Remove the pan from the heat and let the rosemary syrup cool. (This will make a bit more than you need but can be kept in the fridge for quite some time.)
Place 3 tablespoons of the rosemary syrup, the bourbon and seltzer in a shaker filled with ice. Shake vigorously until cold and pour in a rocks glass filled with crushed or cracked ice. Garnish with a fresh rosemary sprig.