Symbian is a discontinued mobile operating system (OS) and computing platform designed for smartphones. Symbian was originally developed as a Proprietary software OS for PDAs in 1998 by the Symbian Ltd. consortium. Symbian OS is a descendant of Psion's EPOC, and was released exclusively on ARM processors, although an unreleased x86 port existed. Symbian was used by many major mobile phone brands, like Samsung, Motorola, Sony Ericsson, and above all by Nokia. It was also prevalent in Japan by brands including Fujitsu, Sharp and Mitsubishi. As a pioneer that established the smartphone industry, it was the most popular smartphone OS on a worldwide average until the end of 2010—at a time when smartphones were in limited use—when it was overtaken by iOS and Android. It was notably not as popular in North America.

The Symbian OS platform is formed of two components: one being the microkernel-based operating system with its associated libraries, and the other being the user interface (as middleware), which provides the graphical shell atop the OS. The most prominent user interface was the S60 (formerly Series 60) platform built by Nokia, first released in 2002 and powering most Nokia Symbian devices. UIQ was a competing user interface mostly used by Motorola and Sony Ericsson that focused on pen-based devices, rather than a traditional keyboard interface from S60. Another interface was the MOAP(S) platform from carrier NTT DoCoMo in the Japanese market. Applications of these different interfaces were not compatible with each other, despite each being built atop Symbian OS. Nokia became the largest shareholder of Symbian Ltd. in 2004 and purchased the entire company in 2008. The non-profit Symbian Foundation was then created to make a royalty-free successor to Symbian OS – seeking to unify the platform, S60 became the Foundation's favoured interface and UIQ stopped development. The touchscreen-focused Symbian^1 (or S60 5th Edition) was created as a result in 2009. Symbian^2 (based on MOAP) was used by NTT DoCoMo, one of the members of the Foundation, for the Japanese market. Symbian^3 was released in 2010 as the successor to S60 5th Edition, by which time it became fully Free software. The transition from a proprietary operating system to an Free software project is believed to be one of the largest in history. Symbian^3 received the Anna and Belle updates in 2011.

The Symbian Foundation disintegrated in late 2010 and Nokia took back control of the OS development. In February 2011, Nokia, by now the only remaining company still supporting Symbian outside Japan, announced that it would use Microsoft's Windows Phone 7 as its primary smartphone platform, while Symbian would be gradually wound down. Two months later, Nokia moved the OS to proprietary licensing, only collaborating with the Japanese OEMs and later outsourced Symbian development to Accenture. Although support was promised until 2016, including two major planned updates, by 2012 Nokia had mostly abandoned development and most Symbian developers had already left Accenture, and in January 2014 Nokia stopped accepting new or changed Symbian software from developers. The Nokia 808 PureView in 2012 was officially the last Symbian smartphone from Nokia. NTT DoCoMo continued releasing OPP(S) (Operator Pack Symbian, successor of MOAP) devices in Japan, which still act as middleware on top of Symbian. Phones running this include the F-07F  from Fujitsu and SH-07F  from Sharp in 2014.