Place the potatoes into a large pot and cover with salted water. Bring to a boil over high heat, then reduce heat to medium-low, cover, and simmer until tender, about 20 minutes. Drain, then run under cold water until cool.

Combine the sour cream and ranch dressing mix in a small bowl until well-blended. Transfer the potatoes, bacon, Cheddar cheese, and green onions to a large bowl. Stir in the sour cream mixture until well-coated. Cover and refrigerate for about 2 hours before serving.