Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9×13-inch baking pan.
Stir cake mix, 1 cup water, vegetable oil, and eggs in a bowl until moistened; beat with an electric mixer on medium speed for 2 minutes. Pour batter into prepared cake pan. Top the batter with rhubarb and sprinkle with sugar and cinnamon; pour 1 cup hot water evenly over the cake.
Bake in the preheated oven until a toothpick inserted into the center of the cake comes out clean or with moist crumbs, 50 minutes to 1 hour.