Joseph Willem Mengelberg (28 March 1871 – 21 March 1951) was a Dutch conductor, famous for his performances of Mahler and Strauss with the Concertgebouw Orchestra.

Mengelberg made commercial recordings in the United States with the New York Philharmonic for Victor (1922–30) and Brunswick (1926-27). In Amsterdam with the Concertgebouw Orchestra he made a series of records issued in various countries on the Columbia and Odeon labels (1926–32) as well as two works recorded for the Dutch branch of Decca in 1935. Mengelberg recorded with the Concertgebouw Orchestra and the Berlin Philharmonic for Telefunken (1937–42). After his death, Philips issued recordings of live performances recorded by Dutch radio services, and these have been reissued by Decca.

In addition to his recordings of Richard Strauss's Ein Heldenleben and Don Juan, Mengelberg left discs of symphonies by Beethoven, Tchaikovski and Brahms, Bach's St Matthew Passion, Gustav Mahler's Symphony No. 4 and the Adagietto from Symphony No. 5.

His most characteristic performances are marked by a tremendous expressiveness and freedom of tempo, perhaps most remarkable in his recording of Mahler's Fourth Symphony but certainly present in the aforementioned St Matthew Passion and other performances as well. These qualities, shared (perhaps to a lesser extent) by only a handful of other conductors of the era of sound recording, such as Wilhelm Furtwängler and Leonard Bernstein, make much of his work unusually controversial among classical music listeners; recordings that more mainstream listeners consider unlistenable will be hailed by others as among the greatest recordings ever made.

Many of his recorded performances, including some live concerts in Amsterdam during World War II, have been reissued on LP and CD. While he was known for his recordings of the German repertoire, Capitol Records issued a powerful, nearly high fidelity recording of César Franck's Symphony in D minor, recorded in the 1940s by Telefunken with the Concertgebouw Orchestra.

Due to the Dutch government's six-year ban on Mengelberg's conducting activities, he made no more recordings after 1945. Some of his performances in Amsterdam were recorded on the innovative German tape recorder, the Magnetophon, resulting in unusually high fidelity for the time.

Sound films of Mengelberg conducting the Concertgebouw Orchestra, during live concerts in Amsterdam, have survived; among these are a 1931 performance of Weber's Oberon overture. His 1939 recording of Bach's St Matthew Passion, while not captured on film, was created on a Philips optical system which used film as the (audio-only) recording medium.