Cook the rotini according to package directions. Drain and rinse with cold water to cool.
In a large bowl, combine the pasta, cherry tomatoes, cucumber, olives, mozzarella, pepperoni, and red onion.
Pour the Italian dressing over the salad and toss to coat evenly.
Refrigerate for at least 30 minutes to allow flavors to develop.
Before serving, taste and adjust seasoning with salt, pepper, or more dressing if needed. Sprinkle with Parmesan if desired.