Birmingham Repertory Theatre, commonly called Birmingham Rep or just The Rep, is a producing theatre based on Centenary Square in Birmingham, England. Founded by Barry Jackson, it is the longest-established of Britain's building-based theatre companies and one of its most consistently innovative.

Today The Rep produces a wide range of drama in its three auditoria (825 seats, 300 seats, and 140 seats), much of which goes on to tour nationally and internationally. The company retains its commitment to new writing and in the five years to 2013 commissioned and produced 130 new plays.

The company's former home, now known as "The Old Rep", is still in use as a theatre.

The origins of The Rep lie with the 'Pilgrim Players', an initially amateur theatre company founded by Barry Jackson in 1907 to reclaim and stage English poetic drama, performing a repertoire that ranged from the 16th century morality play Interlude of Youth to contemporary works by W. B. Yeats. Over the next five years the company staged a total of 28 different productions, aiming to "put before the Birmingham public such plays as cannot be seen in the ordinary way at theatres", but also performing as far afield as London and Liverpool. Their success and reputation led them to turn professional and rename themselves the 'Birmingham Repertory Company' in 1911. By September 1912 Jackson had bought a site in Station Street in Birmingham City Centre and appointed an architect to design what would become Britain's first purpose-built repertory theatre. Construction started the following month and the building – now The Old Rep – opened with a production of Twelfth Night only four months later, on 15 February 1913.

The Rep's stated mission was "to enlarge and increase the aesthetic sense of the public ... to give living authors an opportunity of seeing their works performed, and to learn something from the revival of the classics; in short to serve an art instead of making that art serve a commercial purpose". There had been earlier repertory theatres in Manchester, Glasgow and Liverpool, but the Birmingham project was unique. Previous companies had taken over large commercial theatres and been governed by Boards of Directors; the Birmingham Rep occupied a small-scale auditorium that seated only 464 and was under the sole control of Jackson, whose combination of the roles of patron and artistic director was unique in British theatrical history, allowing the development of a far more imaginative and eclectic programme. Instead of focusing on established star names and popular plays, Jackson's Rep was built around an ensemble cast of young emerging acting talent, performing a repertoire that mixed classics, new writing, experimental productions and the revival of rarely performed works. This was a pivotal development in the establishment of the modern British theatrical landscape, setting the pattern that would later be followed by post-war companies such as the National Theatre and the Royal Shakespeare Company.

The Rep developed its reputation with a series of artistic achievements whose effects would be felt far beyond Birmingham. Thirty-six plays were given their world premieres at The Rep during its first decade, with eight more, including major works by European writers such as Chekhov and Tolstoy, being given their British premieres. John Drinkwater had been one of the original members of the Pilgrim Players and was employed as the Rep's first manager when it opened in 1913. Jackson encouraged his development into a dramatist, presenting a series of his one-act plays and culminating in the 1918 premiere of his first full-length work Abraham Lincoln, whose triumphant success marked a turning point both for playwright and theatre. The Rep's 1923 production of George Bernard Shaw's epic five-play cycle Back to Methuselah gave the company "a profile and stature that set it apart from other repertory theatres in Britain, as well as according it an artistic credibility that no London theatre of the time could match." Of longest-lasting influence however was the production of Cymbeline that opened in Birmingham in April 1923. This was the first performance of Shakespeare to take place in modern dress and it "bewildered" critics, leading to what Jackson happily called "a national and worldwide controversy". The Rep's modern dress production of Hamlet, opening at the Kingsway in August 1925, was the first modern dress staging of a Shakespeare play in London and "was to be the real test" of the technique.

The company also came to be recognised as the country's leading training ground for actors and actresses who would later establish themselves as stars in London, New York or Hollywood. John Gielgud's performance as Romeo with the company in 1924 was his first major role. Peggy Ashcroft made her professional debut with Birmingham seasons in 1926 and 1927. Laurence Olivier's recruitment by The Rep in March 1926 marked his theatrical breakthrough; The Rep was, he later commented, "where I had dreamt of being, where I knew would be found the absolute foundation of any good that I could ever be in my profession".

In 1932, Jackson appointed Herbert M. Prentice as producer at the theatre. Prentice produced and directed over 900 plays before leaving in 1940, as well as writing scripts for some. Some of these were broadcast on BBC radio, and a few by the early BBC television service.