Add the balsamic vinegar and sugar to a small sauce pan. Stir to dissolve the sugar.
Smash the raspberries with a spoon or in a Ziploc bag and add the raspberries to the saucepan.
Heat over medium-high heat until the balsamic begins to simmer, stirring occasionally. Reduce the heat to medium-low. It should be a very gentle simmer.
Stir occasionally until the vinegar has reduced by at least half, about 15-20 minutes. It should coat the back of a cold spoon.
Strain out the raspberry pulp and seeds using a fine mesh sieve. It will thicken as it cools.
Notes
Storage: Store in an airtight container in the refrigerator for up to 2 weeks.