In a small saucepan, combine water and Splenda Stevia Sweetener over medium heat until sweetener is dissolved and liquid is clear (about 2 minutes).
Let cool, then transfer to a jar or plastic container. Cover and refrigerate about 30 minutes, or until cold. (Remaining simple syrup can be stored in the refrigerator for several days.)
To make the drink:
In a tall glass, gently stir together simple syrup, raspberries, mint leaves, and raspberry seltzer water. Add ice.