Preheat the oven to 350°F (175°C). Lightly grease a 9x13 inch baking pan.

Prepare the sugar-free yellow cake mix in a large bowl according to the package directions, using the specified amounts of water, eggs, and oil.

Pour the batter into the prepared pan and smooth the top into an even layer.

Bake for 30 to 35 minutes, or until the cake has golden edges and a toothpick inserted in the center comes out clean.

Remove the cake from the oven and place the pan on a wire rack. Let the cake cool completely to room temperature.

While the cake cools, add the dry sugar-free instant vanilla pudding mix, thawed sugar-free Cool Whip, and the entire can of crushed pineapple with its juice to a medium mixing bowl.

Gently fold the mixture with a spatula until smooth, creamy, and no dry pudding mix remains visible. Let it sit for 3 to 5 minutes so the pudding can thicken slightly.

Once the cake is fully cool, spread the pineapple pudding mixture evenly over the top of the cake, all the way to the edges.

Cover the pan and refrigerate for at least 1 hour before slicing to allow the topping to set.

Slice into 12 even squares and serve chilled.