Armstrong Siddeley was a British engineering group that operated during the first half of the 20th century. It was formed in 1919 and is best known for the production of luxury vehicles and aircraft engines.

The company was created following the purchase by Armstrong Whitworth of Siddeley-Deasy, a manufacturer of luxury motor cars that were marketed to the top echelon of society. After the merge of companies, this focus on quality continued throughout in the production of cars, aircraft engines, gearboxes for tanks and buses, rocket and torpedo motors, and the development of railcars. Company mergers and takeovers with Hawker Aviation and Bristol Aero Engines saw the continuation of the car production which ceased in August 1960.

The company was absorbed into the Rolls-Royce conglomerate which was interested in the aircraft and aircraft engine business. Eventually, the remaining spares and all motor car interests were sold to the Armstrong Siddeley Owners Club Ltd, which now owns the patents, designs, copyrights and trademarks, including the name Armstrong Siddeley.

Considered "an elegant car appropriate for royal use", the "Armstrong Siddeley Saloon" was used by the Prince of Wales (later King Edward VIII) during his 1930 tour of Uganda.

The Siddeley Autocar Company, of Coventry, was founded by John Davenport Siddeley (1866–1953) in 1902. Its products, made for him by a Vickers subsidiary, were heavily based on Peugeots using many Peugeot parts and fitted with English-built bodies. J. D. Siddeley was appointed London sales manager of Vickers Limited's subsidiary Wolseley in early 1905 at the same time as Wolseley purchased the goodwill and patent rights of his Siddeley car. A few months later Herbert Austin left to form his own business and Siddeley was appointed general manager.

Without the consent of the Vickers brothers Siddeley added his own name to the Wolseley nameplate but it was dropped on his departure.

In 1909, J. D. Siddeley resigned from Wolseley and in 1910, he took on management of The Deasy Motor Car Manufacturing Company, Limited. The shareholders were so pleased with his success in that post that on 7 November 1912 they unanimously agreed to change the company's name to The Siddeley-Deasy Motor Car Company Limited. Siddeley's name had been added to the product's radiator earlier in 1912. His cars began to use the slogan "As silent as the Sphinx", sporting a Sphinx as a bonnet mascot.

In April 1919, Siddeley-Deasy was bought out by Armstrong Whitworth Development Company of Newcastle upon Tyne and in May 1919 became Armstrong Siddeley Motors Ltd, a subsidiary with J. D. Siddeley as managing director. In 1927, Armstrong Whitworth merged its heavy engineering interests with Vickers to form Vickers-Armstrongs. At this point, J. D. Siddeley brought Armstrong Siddeley and Armstrong Whitworth Aircraft into his control. In 1928, Armstrong Siddeley Holdings bought Avro from Crossley Motors. Also that year Siddeley partnered with Walter Gordon Wilson, inventor of the pre-selector gearbox, to create Improved Gears Ltd, which later became Self-Changing Gears – the gearbox that should be credited with enabling the marketing tagline "Cars for the daughters of gentlemen".