Add the frozen mixed berries, coconut milk, apple juice, and unflavored protein powder into a blender.
Begin blending at a low speed to prevent the ingredients from splattering. Gradually increase to a high speed until all the ingredients are well combined and the mixture is smooth.
Check the consistency of your smoothie. If it’s too thick, add a bit more apple juice and blend again until you reach your desired thickness.
Pour the smoothie into glasses and serve immediately.