Foursquare, stylized as 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. Each check-in awards the user points and sometimes "badges".

The service was created 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/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.

Dennis Crowley (born June 19, 1976) is an American Internet entrepreneur best known for co-founding the popular social networking sites Dodgeball and Foursquare.

Crowley was born in Medway, Massachusetts and graduated from nearby Xaverian Brothers High School in Westwood, Massachusetts in 1994 and was featured in the front-cover story of Xaverian's seasonal magazine. He received a B.A. in 1998 from Syracuse University's S.I. Newhouse School of Public Communications and a M.P.S. master's degree in 2004 from New York University's Tisch School of the Arts Interactive Telecommunications Program (ITP).

Crowley co-founded Dodgeball with fellow student Alex Rainert in 2003 while attending New York University. Dodgeball was subsequently acquired by Google in 2005, after which Crowley along with Naveen Selvadurai developed a second version of the original Dodgeball service called Foursquare in 2009. Foursquare, offering location-based social networking services via mobile devices, had over 6 million users worldwide as of January 2011, and 10 million users as of June 2011.