Add frozen raspberries and syrup to a high-speed blender. Blend until smooth, pausing to push the raspberry sorbet down if the blades get stuck. If too thick, add 1 tbsp of warm water at a time until it blends smoothly.
Serve immediately, or transfer the raspberry sorbet to a metal loaf tin or other freezer-safe container and place in the freezer for 2-3 hours for a thicker texture.