Bring a large pot of lightly salted water to the boil. Add the pasta and cook until tender, about 8 minutes. Drain and rinse under cold water to cool.
Meanwhile, cook the bacon in a large deep frying pan over medium-high heat until browned and crisp. Remove from the pan and drain on kitchen 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.