Bring a large pot of lightly salted water to a boil. Add the pasta, and cook until tender, about 8 minutes. Drain, and rinse under cold water to cool.
While pasta is cooking, cook the bacon in a large deep skillet over medium-high heat until browned and crisp. Remove from the pan and drain on paper towels.
In a large 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. Crumble bacon over the top just before serving.