Preheat the oven to 350°F (175°C). Grease and flour a 9×5-inch loaf pan.
In a large mixing bowl, combine the yellow cake mix, mashed bananas, eggs, vegetable oil, water, vanilla extract, and ground cinnamon.
Stir the mixture with a spoon or spatula until all ingredients are fully combined and smooth.
Pour the batter evenly into the prepared loaf pan.
Bake in the preheated oven for 45 to 55 minutes, or until a toothpick inserted into the center comes out clean.
Remove from the oven and allow the banana bread to cool in the pan for 10 minutes.
Transfer the bread to a wire rack to cool completely before slicing and serving.