Twin cities are a special case of two cities or urban centres which are born in close geographic proximity and then grow into each other over time. The term Twin Cities in the United States refers specifically to the cities Minneapolis and Saint Paul, Minnesota.

Cities twinned geographically do not necessarily match demographically, economically, or politically.

In the normal course of things, cities which grow into each other's space in this way lose their individual identity and whatever border or barrier still separates them becomes irrelevant until they fuse into one new city. One famous example of this is Budapest in Hungary, which began as two settlements (Buda and Pest) facing each other across the Danube at a strategic fording place along a trade route, with another being London, England, which resulted in the merger of two cities: the City of London and the City of Westminster. But there are twin cities which have been able to resist this final union and have maintained individual identity against the tides of history, economics and demographics.

Twin cities often share an airport, into whose airport code are integrated the initials of both cities; DFW (Dallas-Fort Worth) and MSP (Minneapolis-St. Paul) are well-known examples.