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, both of which are in the midwestern state of Minnesota.

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

In most cases, cities that grow into each other's space in this way lose their individual identities, 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. However, 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 codes are integrated the initials of both cities; DFW (Dallas-Fort Worth), MSP (Minneapolis-St. Paul), and RDU (Raleigh-Durham) are examples.