Combine first 3 ingredients in a large bowl; stir until sugar dissolves.
Place thawed raspberries in a blender; process until smooth. Strain puree through a fine sieve over a bowl. Discard seeds. Add puree to yogurt mixture.
Pour raspberry mixture into the freezer can of an ice-cream freezer; freeze according to manufacturer's instructions. Spoon into a freezer-safe container; cover and freeze 1 hour or until firm. Garnish with fresh raspberries, if desired.
MyRecipes is working with Let's Move!, the Partnership for a Healthier America, and USDA's MyPlate to give anyone looking for healthier options access to a trove of recipes that will help them create healthy, tasty plates. For more information about creating a healthy plate, visit www.choosemyplate.gov.