Add the dry instant pistachio pudding mix and the undrained crushed pineapple, including all of its juice, to a large mixing bowl. Stir until the powder is fully moistened and the mixture begins to thicken.

Gently fold the thawed whipped topping into the pineapple and pudding mixture. Continue folding just until the color and texture look evenly combined, taking care not to stir vigorously.

Cover the bowl and refrigerate the dessert for at least 1 hour. For the firmest texture and best flavor, chill it for 1 to 2 hours or preferably overnight.

Serve the pistachio dessert cold in small bowls or dessert cups. If desired, add chopped pistachios, a maraschino cherry, or a small spoonful of whipped topping just before serving.