友好城市（或姊妹城市）指将地域上或政治上无关的城镇或城市配对起来，以期达到增加居民或文化交流的目的。友好城市通常有类似的规模或是其他特征，但也并非所有的例子都是如此。有时还有更大的区域进行这种配对，如中华人民共和国的苏州和意大利的威尼斯。

友好城市的概念被提出跟普通人之间的笔友关系有所相似之处，不过却是整个城市或城镇的规模。事实上，姊妹城市之间时常会互相提供交换学生，以及经济或文化上的交流或合作。

Twin towns or sister cities are a form of legal or social agreement between towns, cities, counties, oblasts, prefectures, provinces, regions, states, and even countries in geographically and politically distinct areas to promote cultural and commercial ties. The modern concept of town twinning, conceived after the Second World War in 1947, was intended to foster friendship and understanding between different cultures and between former foes as an act of peace and reconciliation, and to encourage trade and tourism. In recent times, town twinning has increasingly been used to form strategic international business links between member cities.