Bring a lg pot of lightly salted water to a boil. Add the pasta and cook until tender, about 8 minutes. Drain and rinse under cold water.
Meanwhile, cook the bacon in a lg deep skillet over medium-high heat until browned and crisp. Remove from pan and drain on paper towels.
In a lg bowl, stir together the Ranch dressing, onion, and tomatoes. Mix in the cooled pasta. (The pasta will absorb some of the dressing, so don't worry if it seems like too much.) Refrigerate for several hours or overnight.