Symbian is a mobile operating system (OS) and computing platform designed for smartphones and currently maintained by Accenture. The Symbian platform is the successor to Symbian OS and Nokia Series 60; unlike Symbian OS, which needed an additional user interface system, Symbian includes a user interface component based on S60 5th Edition. The latest version, Symbian^3, was officially released in Q4 2010, first used in the Nokia N8. In May 2011 an update, Symbian Anna, was officially announced, followed by Nokia Belle (previously Symbian Belle) in August 2011.

Symbian OS was originally developed by Symbian Ltd. It is a descendant of Psion's EPOC and runs exclusively on ARM processors, although an unreleased x86 port existed.

Some estimates indicate that the number of mobile devices shipped with the Symbian OS up to the end of Q2 2010 is 385 million.

By April 5, 2011, Nokia released Symbian under a new license and converted to a proprietary model as opposed to an open source project.

On February 11, 2011, Nokia announced that it would migrate from Symbian to Windows Phone 7. Nokia CEO Stephen Elop announced Nokia's first Windows phones at Nokia World 2011: the Lumia 800 and Lumia 710. These phones were launched on November 14, 2011. On June 22, 2011 Nokia made an agreement with Accenture for an outsourcing program. Accenture will provide Symbian-based software development and support services to Nokia through 2016; about 2,800 Nokia employees became Accenture employees as of October 2011. The transfer was completed on September 30, 2011.