Prepare the cake mix according to package directions in a 9x13-inch pan. Let cool slightly.
Using the handle of a wooden spoon or skewer, poke holes evenly all over the cake.
Dissolve the strawberry gelatin in 1 cup boiling water. Stir in ½ cup cold water.
Pour the gelatin mixture evenly over the cake, letting it seep into the holes.
Pour the sweetened condensed milk over the cake, filling the holes as much as possible.
Cover and refrigerate for at least 4 hours, or overnight for best results.
Spread whipped topping evenly over the chilled cake. Garnish with fresh sliced strawberries before serving.