Combine sugar and 3/4 cup water in a saucepan over medium heat. Cook until sugar is dissolved, about 2 minutes. Remove syrup from heat.
Blend together berries and 1 cup water. Strain through a fine-mesh sieve into a large pitcher.
Add syrup, lemon juice and 6 cups cold water to pitcher; stir to combine. Drop a few raspberries in each glass to garnish, if desired.