Preheat the oven to 350°F (175°C). Grease and flour a 9×13-inch baking pan.
In a large mixing bowl, combine the yellow cake mix, mashed bananas, eggs, vegetable oil, water, and vanilla extract.
Using an electric mixer or whisk, beat the ingredients together on medium speed for 2 minutes until well combined and smooth.
Pour the batter evenly into the prepared baking pan and spread it out with a spatula.
Bake in the preheated oven for 30 to 35 minutes or until a toothpick inserted into the center comes out clean.
Remove the cake from the oven and allow it to cool completely in the pan on a wire rack before slicing and serving.