In a large mixing bowl, combine the instant pistachio pudding mix and the undrained crushed pineapple.

Stir the mixture until the pudding powder is completely dissolved and well incorporated with the pineapple juice.

Gently fold in the whipped topping until no white streaks remain and the mixture is a uniform light green color.

Add the miniature marshmallows and chopped walnuts; fold gently to distribute evenly throughout the salad.

Cover the bowl and refrigerate for at least 1 hour to allow the flavors to meld and the marshmallows to soften.

Transfer to a serving bowl and garnish with maraschino cherries if desired before serving.