The Plessey Company plc (also known as The Plessey Company Ltd) was a British electronics, defence and telecommunications company. It originated in 1917, growing and diversifying into electronics. It expanded after World War II by acquisition of companies and formed overseas companies. It was listed on the London Stock Exchange and was a constituent of the FTSE 100 Index.

In 1951, the Electronics Division started at Ilford and in 1955 the Electronics and Equipment Group was established. Around circa 1961, The Plessey Company set up four other businesses: Telecommunications Division, Plessey Avionics and Communications, Plessey Radar (Weybridge) and Plessey Marine. In 1988, Plessey and The General Electric Company (GEC) merged their telecom units to form GEC-Plessey Telecommunications (GPT), with the ownership then split with Siemens AG in 1989.