Boil the rotini according to the package instructions, adding a pinch of salt to the water. Once cooked, drain and rinse under cold water to cool.
In a large bowl, combine the cooled pasta, cucumber, tomatoes, feta cheese, black olives, and red onion. Drizzle with the Italian dressing and toss until everything is well coated.
For the best flavor, let the salad chill in the refrigerator for a couple of hours before serving. This allows the flavors to meld together beautifully.