Reginald Joseph Mitchell (20 May 1895 – 11 June 1937) was a British aircraft designer who worked for the Southampton aviation company Supermarine from 1916 until 1936. He is best known for designing racing seaplanes, such as the Supermarine S.6B, and for leading the team that designed the Supermarine Spitfire.

Born in Butt Lane, Staffordshire, Mitchell attended Hanley High School and afterwards worked as an apprentice at a locomotive engineering works, whilst also studying engineering and mathematics at night. In 1916 he moved to Southampton to join Supermarine. He was appointed Chief Engineer in 1920 and Technical Director in 1927. Between 1920 and 1936 he designed 24 aircraft, which included flying boats and racing seaplanes, light aircraft, fighters and bombers. From 1925 to 1929 he worked on a series of racing seaplanes, built by Supermarine to compete in the Schneider Trophy competition, the final entry in the series being the Supermarine S.6B. The S.6B won the trophy in 1931. Mitchell was authorised by Supermarine to proceed with a new design, the Type 300, which went on to become the Spitfire.

In 1933, Mitchell underwent surgery to treat rectal cancer. He continued to work and earned his pilot's licence in 1934, but in early 1937, he was forced by a recurrence of the cancer to give up work. After his death that year, he was succeeded as chief designer at Supermarine by Joseph Smith.

Reginald Joseph Mitchell was born on 20 May 1895 at 115 Congleton Road, Butt Lane, in Staffordshire, England. He was the second eldest of five children, and the eldest of three brothers. His father Herbert Mitchell was a Yorkshireman who became headmaster of three Staffordshire schools in the Stoke-on-Trent area, before he retired from teaching. He then helped to establish a printing business, Wood, Mitchell and C. Ltd, in Hanley. Herbert Mitchell's wife Eliza Jane Brain was the daughter of a cooper. When Reginald was a child, the family lived in Normacot, now a suburb of Stoke-on-Trent.

Reginald (known to his family as "Reg") attended Queensberry Road Higher Elementary School from the age of eight, before moving on to Hanley High School. There he developed an interest in making and flying model aircraft. In 1911, after leaving school at the age of 16, he worked as an apprentice for Kerr Stuart & Co. of Fenton, a railway engineering works. After completing his apprenticeship he worked in the drawing office at Kerr Stuart, whilst studying engineering and mathematics at a local technical college, where he displayed a talent for mathematics.

After leaving Kerr Stuart in 1916, Mitchell worked for a period as a part-time teacher. He applied to join the armed forces on two occasions, but was on each occasion rejected because of his training as an engineer.

Reginald had a nephew; J. W. Mitchell, an artist who painted several scenes involving his uncle's spitfires, profits from his sales of his work were donated to the RAF Benevolent Fund.

In 1916, Mitchell joined the Supermarine Aviation Works at Southampton, possibly for a probationary period. Since its formation in 1912, the company had specialised in building flying boats, producing its first aircraft, the Pemberton-Billing P.B.1, in 1914. During the First World War, Supermarine was taken over by the British Government, and during this period the company produced the first British single-seat flying boat fighter, the Supermarine Baby.