Place the sugar, lemon juice and zest in a saucepan over a medium heat, stirring from time to time, until the sugar dissolves completely, about five minutes.
Put the raspberries and the sugar syrup in a blender and puree.
Strain through a fine mesh sieve to remove the seeds and stir in the finely chopped fresh mint.
Transfer to a sterilised glass jar or bottle.
Delicious poured over desserts or as a base of a cocktail, or add sparkling water for a refreshing non-alcoholic drink.