Preheat the oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners or grease the muffin cups.
In a large mixing bowl, combine the yellow cake mix, mashed bananas, eggs, vegetable oil, water, and vanilla extract.
Stir the ingredients together until just combined and smooth, being careful not to overmix.
Divide the batter evenly among the 12 muffin cups, filling each about 2/3 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 tin for 5 minutes.
Transfer the muffins to a wire rack to cool completely before serving.