Duolingo, Inc. is an American educational technology company that produces learning apps and provides language certification. Duolingo offers courses on 43 languages, ranging from English, French, and Spanish to less commonly studied languages such as Welsh, Irish, and Navajo, and even constructed languages such as Klingon. It also offers courses on music, math, and chess. The learning method incorporates gamification to motivate users with points, rewards and interactive lessons featuring spaced repetition. The app promotes short, daily lessons for consistent, phased practice.

Duolingo also offers the Duolingo English Test, an online language assessment, and Duolingo ABC, a literacy app designed for children. The company follows a freemium model, where some content is provided for free with advertising, and users can pay for ad-free services which provide additional features.

The idea of Duolingo was formulated in 2009 by Carnegie Mellon University professor Luis von Ahn and his Swiss-born post-graduate student Severin Hacker. Von Ahn had sold his second company, reCAPTCHA, to Google and, with Hacker, wanted to work on an education-related project. Von Ahn stated that he saw how expensive it was for people in his community in Guatemala to learn English. Hacker (co-founder and current CTO of Duolingo) believed that "free education will really change the world" and wanted to provide an accessible means for doing so. He was recognized by the National Inventors Hall of Fame for his contributions to language learning and technological development. The Duo mascot is a green owl because co-founder Severin Hacker hates the color green.

The project was originally financed by von Ahn's MacArthur fellowship and a National Science Foundation grant. The founders considered creating Duolingo as a nonprofit organization, but von Ahn judged this model unsustainable. Its early revenue stream, a crowdsourced translation service, was replaced by a Duolingo English Test certification program, advertising, and subscription.

In October 2011, Duolingo announced that it had raised $3.3 million from a Series A round of funding, led by Union Square Ventures, with participation from author Tim Ferriss and actor Ashton Kutcher's investing firm A-Grade Investments. Duolingo launched a private beta on November 30, 2011, and accumulated a waiting list of more than 100,000 people by December 13. It launched to the general public on June 19, 2012, at which point the waiting list had grown to around 500,000.

In September 2012, Duolingo announced that it had raised a further $15 million from a Series B funding round led by New Enterprise Associates, with participation from Union Square Ventures. In November 2012, Duolingo released an iPhone app, followed by an Android app in May 2013, at which time Duolingo had around 3 million users. By July 2013, it had grown to 5 million users and was rated the No. 1 free education app in the Google Play Store.

In February 2014, Duolingo announced that it had raised $20 million from a Series C funding round led by Kleiner Caufield & Byers, with prior investors also participating. At this time, it had 34 employees, and reported about 25 million registered users and 12.5 million active users; it later reported a figure closer to 60 million users.

In June 2015, Duolingo announced that it had raised $45 million from a Series D funding round led by Google Capital, bringing its total funding to $83.3 million. The round valued the company at around $470 million, with 100 million registered users globally. In April 2016, it was reported that Duolingo had more than 18 million monthly users.