Bring water, sugar and lemon juice to a boil in a medium saucepan, stirring to dissolve sugar. Remove from heat and let cool completely.

Place raspberries and cooled lemon syrup in a blender and puree. Strain raspberry mixture through fine-mesh sieve to remove seeds. Discard seeds.

Pour strained mixture into 13 x 9-inch glass or metal baking pan. Place in freezer and chill about 1 hour or until edges begin to freeze. Using a fork, scrape mixture towards center of pan. Repeat every 30 minutes until mixture is frozen and flaky, about 1 1/2 hours longer. Cover and keep frozen until ready to serve. Granita can be made up to 2 days ahead.