Add all ingredients to a medium saucepan and heat over medium-low heat. Whisk continuously until the sugar is dissolved and everything is smooth and combined.
Remove from heat and allow to cool at room temperature for 15 minutes. Transfer the peanut butter syrup to a syrup bottle or mason jar and store in the fridge for up to 2 weeks.
Notes
This makes about 2.25 cups of peanut butter syrup.