Marcussen & Søn, known as Marcussen and previously as Marcussen & Reuter, is a Danish firm of pipe organ builders.
They were one of the first firms to go back to classical organ-building techniques, and have been producing mechanical-action organs since 1930. Aside from their many instruments in Denmark, they have built organs in northern Germany, Sweden, Finland, the Netherlands, Great Britain, South Africa, Japan, and the United States.
Jürgen Marcussen (1781–1860) founded the organ-building company in 1806. They used the name Marcussen & Reuter from 1826 to 1848, when the name became Marcussen & Søn after the founder's son, Jürgen Andreas Marcussen, joined the firm. The company has been based in a house in the small town of Åbenrå, in southern Jutland, since 1830. Several organs built in Scandinavia and North Germany in their first decades are still in use today, the oldest dating from 1820.
Johannes Lassen Zachariassen (1864–1922), a grandson of the founder's daughter, took over the firm from 1902 to 1922. The firm's work was still based at this stage on the Baroque organ-building tradition, but from about 1900, in common with nearly all other organ-builders, they began making use of pneumatics, electricity, and other innovations popular at the time, typified by the organs of Cavaillé-Coll.
This new development did not last long. Following an 1925 organ conference in Hamburg and Lübeck, Marcussen was one of the first builders of the Organ Reform Movement, returning to the sonic, structural, and technical principles of the North-European Baroque organ, employing ultra-low wind pressures and high-pitched, vertical principal choruses characteristic of the instruments of Gottfried Silbermann and Arp Schnitger.