Twilio (/ˈtwɪliːoʊ/) is a cloud communications platform as a service (PaaS) company based in San Francisco, California. Twilio allows software developers to programmatically make and receive phone calls and send and receive text messages using its web service APIs. Twilio's services are accessed over HTTP and are billed based on usage.

Twilio was founded in 2007 by Jeff Lawson, Evan Cooke, and John Wolthuis and was originally based in both Seattle, Washington, and San Francisco, California.

Twilio's first major press coverage, in November 2008, was the result of an application built by Jeff Lawson to Rickroll people, which investor Dave McClure used on TechCrunch founder and editor Michael Arrington as a prank. A few days later on November 20, 2008, the company launched Twilio Voice, an API to make and receive phone calls completely hosted in the cloud. Twilio's text messaging API was released in February 2010, and SMS shortcodes were released in public beta in July 2011.

Twilio raised approximately $103 million in venture capital growth funding. Twilio received its first round of seed funding in March 2009 for an undisclosed amount, rumored to be around $250,000, from Mitch Kapor, The Founders Fund, Dave McClure, David G. Cohen, Chris Sacca, Manu Kumar, from K9 Ventures and Jeff Fluhr. Twilio's first A round of funding was led by Union Square Ventures for $3.7 million and its second B round of funding, for $12 million, was led by Bessemer Venture Partners. Twilio received $17 million in a Series C round in December 2011 from Bessemer Venture Partners and Union Square Ventures. In July 2013 Twilio received another $70 million from Redpoint Ventures, Draper Fisher Jurvetson (DFJ) and Bessemer Venture Partners.

Twilio was ranked 8th on Fast Company’s Most Innovative Companies of 2017 list. Their work for giving apps a voice was noted in their ranking.

Twilio is known for its use of platform evangelism to acquire customers. An early example is GroupMe, which was founded in May 2010 at the TechCrunch Disrupt hackathon and uses Twilio's text messaging product to facilitate group chat. It raised $10.6 million in venture funding in January 2011.

Following the success of the TechCrunch Disrupt hackathon Seed accelerator 500 Startups announced the Twilio Fund, a $250,000 "micro-fund" to provide seed money to startups using Twilio in September 2010.

In February 2015, Twilio acquired Authy, a Y Combinator-backed startup that offers two-factor authentication services to end users, developers and enterprises.

In September 2016, Twilio acquired Tikal Technologies, the development team behind the Kurento WebRTC open source project, for $8.5 million.

Twilio uses Amazon Web Services to host telephony infrastructure and provide connectivity between HTTP and the public switched telephone network (PSTN) through its APIs.

Twilio follows a set of architectural design principles to protect against unexpected outages, and received praise for staying online during the widespread Amazon Web Services outage in April 2011.

Twilio supports the development of open-source software and regularly makes contributions to the open-source community. In June 2010 Twilio launched OpenVBX, an open-source product that lets business users configure phone numbers to receive and route phone calls. One month later, Twilio engineer Kyle Conroy released Stashboard, an open-source status dashboard written in the Python programming language that any API or software service can use to display whether their service is functioning properly. Twilio also sponsors Localtunnel, created by now ex-Twilio engineer Jeff Lindsay, which enables software developers to expose their local development environment to the public internet from behind a NAT.

Twilio lists a number of other open-source projects on their website including: