Twin cities are a special case of two cities or urban centres which are founded 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 as 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. 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.

In some cases, such as Albury/Wodonga in Australia, the two cities are permanently divided by a state border, often one that strictly adheres to a geographical landmark, such as the Murray River that divides New South Wales from Victoria, and thus, Albury from Wodonga.