Heat the balsamic vinegar in a pot over medium-high heat until it begins to simmer. Reduce the heat to medium-low and stir continuously until it thickens up.
Once the mixture is slightly thick, remove the pot from the heat and let it cool down slightly.
Pour the balsamic glaze into a container and place it in the fridge for at least 20 minutes to chill.
In a large mixing bowl, add the watermelon chunks, avocado chunks, feta, and chopped mint leaves. Toss with a wooden spoon to evenly distribute.
Transfer the watermelon feta mint salad to a serving container, if desired, and drizzle the balsamic glaze over top.