The company said in a media release on Monday that it was moving to Ubuntu "as part of its effort to drive innovation in the network and cloud".
Canonical, owned by South African millionaire Mark Shuttleworth, released the Ubuntu distribution in October 2004 and, has since then, issued new versions every six months.
AT&T, which has existed in its current form since 2005, said Canonical would provide the Ubuntu operating system and engineering support for AT&T’s cloud, network and enterprise applications.
It said the choice was made due to the "demonstrated innovation, and performance as the leading platform for scale-out workloads and cloud".
"We're re-inventing how we scale by becoming simpler and modular, similar to how applications have evolved in cloud data centres. Open source and OpenStack innovations represent a unique opportunity to meet these requirements and Canonical's cloud and open source expertise make them a good choice for AT&T."
John Zannos, vice-president, Cloud Alliances and Business Development at Canonical, was quoted thus: "This is important for Canonical. AT&T's scalable and open future network utilises the best of Canonical innovation.
"AT&T selecting us to support its effort in cloud, enterprise applications and the network provides the opportunity to innovate with AT&T around the next generation of the software-centric network and cloud solutions.
"Ubuntu is the operating system of the cloud and this relationship allows us to bring our engineering expertise around Ubuntu, cloud and open source to AT&T.'