A twin building is any structure or built object which looks almost indistinguishable from another structure next to it. Examples include neighboring housing units that follow the same design, as well as twin towers, repeating infrastructural elements, etc.

A twin can be a mirror image of its counterpart with a different spatial orientation, as long as they are otherwise alike in features and dimensions.

Twin buildings can be separated by a street, but identical structures which are so far separated that they are not visible together are not considered twins. Single structures with twin elements (such as twin steeples on a church) are not listed as twin buildings.