Richard Stewart Addinsell (13 January 1904 – 14 November 1977) was an English composer, known early in his career for his music for the theatre in collaboration with Clemence Dane, later for his film music – including his best-known piece, the Warsaw Concerto, composed for the 1941 Dangerous Moonlight – and subsequently for his musical collaboration with the lyricist and performer Joyce Grenfell.

Richard Addinsell was born in Woburn Square, London, on 13 January 1904, the younger son of William Arthur Addinsell, who was a chartered accountant, and his wife, Annie Beatrice née Richards. Addinsell was educated at home before attending Hertford College, Oxford, to study law but going down after 18 months. He became interested in music as an alternative career. In 1925 he enrolled at the Royal College of Music (RCM) but lasted only two terms before leaving, again without obtaining any formal qualification.

While still a student at the RCM, Addinsell (billed as Dick, rather than Richard) was contributing the music for a revue, The Charlot Show of 1926, with Noel Gay. A revue for Fred Karno followed in 1927, again with Gay,

In 1928 Addinsell first collaborated with the author Clemence Dane. The work was Adam's Opera, described by the two collaborators as "a tragic pantomime, set to music and written in nursery rhyme". It was presented at The Old Vic in December 1928. The critics were equivocal about Dane's text, but Addinsell's music was described by several as "catchy", and the theatrical newspaper The Era called it "By far the best part of the entertainment":

In 1929 Addinsell completed his informal education by touring Europe to visit major theatrical and musical centres such as Berlin and Vienna. He wrote the music for J. B. Priestley and Edward Knoblock's 1931 stage adaptation of Priestley's 1929 novel The Good Companions, in which John Gielgud played a young musician, composer of the music actually provided by Addinsell.

In 1932 Addinsell wrote the incidental music for a Broadway conflation of Alice in Wonderland and Through the Looking Glass by Eva Le Gallienne, starring Josephine Hutchinson. His music was praised, but the meagre orchestral forces at the composer's disposal prompted Burns Mantle to call for the score to "be elaborated and orchestrated for a full-sized band", when sections of it would be more impressive. The following year, also for New York, Addinsell composed the music for another "play in words and music" with Dane – Come of Age – which opened at the Maxine Elliott Theatre in January 1934 with Judith Anderson in the lead.

Dane then wrote Moonlight is Silver for which Addinsell composed the music. The piece opened in the West End in September 1934 starring Gertrude Lawrence and Douglas Fairbanks Jr.; Lawrence and Fairbanks recorded a scene from the play for His Master's Voice, with Addinsell's music included in the background throughout, until at the end, Lawrence sings his setting of the title song. Next, Dane adapted Edmond Rostand's play L'Aiglon for Le Gallienne; it opened at the Broadhurst Theatre in New York in November 1934, with music by Addinsell. By 1934 Addinsell was well enough known for the BBC to devote a whole programme to his music.

Addinsell's last theatre compositions of the 1930s were The Happy Hypocrite (1936, Dane's dramatisation of Max Beerbohm's short story of the same name) and incidental music for a 1938 production of The Taming of the Shrew starring Edith Evans and Leslie Banks at the New Theatre. His association with the cinema began in 1936, although he had been in Hollywood in 1933 in connection with a planned film starring Francis Lederer, but the film was never made. Addinsell's introduction to writing for films came in 1936 when his contacts with Fairbanks (who starred) and Dane (co-author) led to his being invited to compose the music for The Amateur Gentleman.