The Plessey Company plc 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 1989, it was taken over by a consortium formed by GEC and Siemens which split the assets of the Plessey group.

The majority of Plessey's defence assets were amalgamated into BAE Systems in 1999 when British Aerospace merged with the defence arm of GEC, Marconi Electronic Systems (MES). The Plessey Microsystems division was the subject of a management buyout in 1988 becoming Radstone Technology, which survives today as part of Abaco Systems based in Towcester, Northamptonshire. The bulk of Plessey's telecommunications assets were acquired by Ericsson through its 2005 acquisition of Marconi Communications, a successor company of GEC.

The Plessey company was founded in 1917 in Marylebone, central London. The original shareholders were Thomas Hurst Hodgson, C. H. Whitaker, Raymond Parker and his brother Plessey Parker. A talented German engineer, William Oscar Heyne, was employed by the company. Heyne later became the managing director and chairman of Plessey and was one of the key figures in the development of Plessey during the 1920s and 1930s. The company moved to Cottenham Road in Ilford early in 1919. In 1925, the original company was wound up and a new one was formed with a greater share capital. Most of the early work carried out by the company was in mechanical engineering.

An early customer of Plessey was a galvanising company called British Electro Chemists. One of that company's shareholders was Byron G. Clark, an American, who was also a business associate of T. H. Hodgson, one of the founders of Plessey. The Clark family would eventually dominate the management of Plessey for most of its history. Byron's son Allen George Clark joined the company in 1921, and went on to become a driving force behind the development of Plessey, followed later by his sons John Allen Clark, and Michael William Clark, both of whom rose to prominent positions in the company.

During the 1920s Plessey began to diversify into electrical manufacturing. Important contracts included the manufacture of early radios for Marconi and the production of telephones for the General Post Office. In order to increase production, Plessey moved to Vicarage Lane, Ilford, in 1923. In 1929, the television pioneer John Logie Baird had his first production televisions made by Plessey. The company also produced the first British-made portable radio in the same year.

The manufacture of electrical components became an area of growth for Plessey. A vast array of components was manufactured, many under licence from overseas companies. Plessey became one of the largest manufacturers in this field as the radio and television industries grew. In 1936/7, turnover was more than £1 million and Plessey became a public company on 17 March 1937.

Aircraft components was another market into which the company diversified. In 1936, Plessey obtained a number of important manufacturing licences from American companies such as Breeze Corporation for aircraft multi-pin electrical connectors, Federal Laboratories for Coffman starters (an explosive cartridge device used to start aircraft engines), and Pump Engineering Services Corporation for the manufacture of Pesco fuel pumps. Plessey went on to produce large numbers of these fuel pumps for Rolls-Royce Merlin engines, and in 1940 the fuel pump for Britain's first jet engine was also supplied by Plessey.

During World War II, Plessey produced a vast array of components and equipment for the war effort, including shell cases, aircraft parts, and radio equipment such as the R1155 (receiver), and T1154 (transmitter). In all, 161,500 pieces of wartime electronic equipment were produced.