Psion PLC was a designer and manufacturer of mobile handheld computers for commercial and industrial uses. The company was headquartered in London, England, with major operations in Mississauga, Ontario, Canada, and other company offices in Europe, the United States, Asia, Latin America, and the Middle East. It was a public company listed on the London Stock Exchange (LSE: PON) and was once a constituent of the FTSE 100 Index.

Psion's operational business was formed in September 2000 from a merger of Psion and Canadian-based Teklogix Inc., and was a global provider of solutions for mobile computing and wireless data collection. The Group's products and services included rugged mobile hardware, secure software and wireless networks, professional services, and support programs. Psion worked with its clients in the area of burgeoning technologies, including imaging, voice recognition, and radio-frequency identification (RFID). They had operations worldwide in 14 countries, and customers in more than 80 countries.

Formed in 1980, Psion first achieved success as a consumer hardware company that developed the Psion Organiser and a wide range of more sophisticated clamshell personal digital assistants (PDAs). Psion either closed or disposed of all its prior operations and then focused on rugged mobile computing systems. It withdrew from the consumer device market in 2001. Motorola Solutions announced in June 2012 that it had agreed to acquire Psion for $200 million.

Psion was established in 1980 as a software house with a close relationship with Sinclair Research. The company developed games and other software for the ZX81 and ZX Spectrum home computers, released under the Sinclair/Psion brand. Psion's games for the ZX Spectrum included Chequered Flag and Flight Simulation.

Early software releases for the ZX Spectrum included titles such as VU-Calc, VU-File and VU-3D, along with dozens of other titles.

The company name is an acronym standing for "Potter Scientific Instruments", after the company's founder, David Potter. The acronym PSI was already in use elsewhere in the world so "ON" was added to make the name unique. Potter remained managing director until 1999 and was chairman of the company until late 2009.

In early 1983, Sinclair approached Psion regarding the development of a suite of office applications for the forthcoming Sinclair QL personal computer. Psion were already working on a project in this area, and when the QL was launched in 1984 it was bundled with Quill, Archive, Abacus and Easel; respectively a word processor, database, spreadsheet, and business graphics application. These were later ported to DOS and made available for the IBM PC and ACT's Sirius and Apricot computers, collectively called PC-Four, or Xchange in an enhanced version. Xchange was also available for ICL's One Per Desk computer, which was based on the QL.

In 1984, Psion first entered the hardware market with the Psion Organiser, an early handheld computer, in appearance resembling a pocket calculator with an alphanumeric computer keyboard. In 1986, the vastly improved Psion Organiser II was released, and was assembled by Speedboard Assembly Services. Its success led the company into a decade long period of Psion Computer and operating system development. It included the simple-to-use Open Programming Language (OPL) for database programming, which sparked a large independent software market.