Preheat the oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners or grease the cups.
In a large mixing bowl, combine the yellow cake mix, mashed bananas, eggs, vegetable oil, water, and vanilla extract.
Stir the mixture with a spoon or spatula until all ingredients are well combined and the batter is smooth.
Divide the batter evenly among the 12 muffin cups, filling each about 3/4 full.
Bake in the preheated oven for 18 to 22 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Remove the muffins from the oven and allow them to cool in the pan for 5 minutes.
Transfer the muffins to a wire rack to cool completely before serving.