Foursquare is a location-based social networking website for mobile devices, such as smartphones. Users "check in" at venues using a mobile website, text messaging or a device-specific application by selecting from a list of venues the application locates nearby. Location is based on GPS hardware in the mobile device or network location provided by the application, and the map is based on data from the OpenStreetMap project. Each check-in awards the user points and sometimes "badges". The user who checks in the most often to a venue becomes the "mayor," and users regularly vie for "mayorships."

The service was created in late 2008 and launched in 2009 by Dennis Crowley and Naveen Selvadurai. Crowley had previously founded the similar project Dodgeball as his graduate thesis project in the Interactive Telecommunications Program (ITP) at New York University. Google bought Dodgeball in 2005 and shut it down in 2009, replacing it with Google Latitude. Dodgeball user interactions were based on SMS technology, rather than an application.

Foursquare is the second iteration of the same idea, that people can use mobile devices to interact with their environment. As of April 2012, the company reported it had 20 million registered users. The company was expected to pass 750 million check-ins before the end of June 2011, with an average of about 3 million check-ins per day. Male and female users are equally represented and also 50 percent of users are outside the US.Support for French, Italian, German, Spanish, and Japanese was added in February 2011. Support for Indonesian, Korean, Portuguese, Russian, and Thai was added in September 2011.Support for Turkish was added in June 2012.