37signals is a privately held web application company based in Chicago, Illinois. The firm was co-founded in 1999 by Jason Fried, Carlos Segura, and Ernest Kim as a web design company. Segura left in 2000 and Kim left in 2003, leaving Fried as the only remaining founder.

Since mid-2004, the company's focus has shifted from web design to web application development. Its first commercial application was Basecamp; this was followed by Backpack, Campfire, and Highrise. It maintains two freeware web applications, Ta-Da List and Writeboard. The open source web application framework Ruby on Rails was initially created for internal use at 37signals, before being publicly released in 2004.

The company is named for the 37 radio telescope signals identified by astronomer Paul Horowitz as potential messages from extraterrestrial intelligence. In 2006, co-founder Jason Fried was named to the MIT Technology Review TR35 as one of the top 35 innovators in the world under the age of 35.

The company's initial focus was on website redesign, including work for Panera Bread and Shopping.com. The Meetup.com website was designed by web design service, whereby a single web page would be redesigned for a set fee in one week.^{[citation needed]}