Geirr Tveitt (born Nils Tveit; 19 October 1908 – 1 February 1981) was a Norwegian composer and pianist. Tveitt was a central figure of the national movement in Norwegian cultural life during the 1930s.

Tveitt was born in Bergen, where his father briefly worked as a teacher. His parents were Håkonson Lars Tveit (1878–1951) and Johanna Nilsdotter Heradstveit (1882–1966). His family were of farmer stock, and still retained Tveit, their ancestral land in Kvam Municipality – a secluded area on the Hardangerfjord. The Tveit family would relocate to Drammen in the winter to work, but return to Hardanger in the summer to farm. Tveitt had originally been christened Nils, but following his increasing interest in Norwegian heritage, he thought the name was not Norwegian enough and changed it to Geir. He later added an extra r to his first name and an extra t to Tveit to indicate more clearly to non-Norwegians the desired pronunciation of his name. It was during his childhood summers in Hardanger that Tveitt gained knowledge of the rich folk-music traditions of the area. Historically, Hardanger's relative isolation allowed for the development of a unique musical culture, with which Tveitt became infatuated. Tveitt would then go on to learn to play both the violin and the piano. After being encouraged by Norwegian composer Christian Sinding, Tveitt decided to try his hand at writing music.

In 1928 Tveitt left Norway to study. He headed for Germany – to Leipzig and its Conservatory, which had been the hub of European musical learning and culture for a long time. There he studied composition with Hermann Grabner and Leopold Wenninger, and piano with Otto Weinreich. However, since he lacked funds, he had to rely on translation work and donations to support himself. In 1930 he'd get 12 Two-part Inventions in Lydian, Dorian and Phrygian accepted for publishing by Breitkopf & Hartel. The following year the Leipzig Radio Orchestra premiered Tveitt's first Piano Concerto.

Tveitt had become increasingly frustrated with the teaching in Leipzig so in 1932 Tveitt headed on to Paris. Here he obtained lessons from some other important composers of the times: Arthur Honegger and Heitor Villa-Lobos. He further managed to enroll in the classes of Nadia Boulanger. Tveitt also made a visit to Vienna, where he was able to study for some time with Austrian composer Egon J. Wellesz, a former pupil of Arnold Schoenberg. Tveitt made one last educational stopover in Paris in 1938 before heading back home to Norway for work. Compared to other Norwegian composers contemporary with Tveitt, he had perhaps the most diverse education – and he had already started to make a name for himself. His writings and compositions made quite a stir amongst the establishment in Oslo. In the years leading up to World War II, Tveitt derived most of his income working as music critic to Sjofartstidende (The Naval Times). Tveitt's highly opinionated reviews contributed to him getting strong opponents – one of these was the Norwegian composer, Pauline Hall.

Once World War II ended, Tveitt brought his scores with him to Europe, travelling extensively – often performing his own piano works with similar works by other composers, e.g. Grieg and Chopin. Many of the concerts were great successes, especially the 1947 concert in Paris. There Tveitt premiered his Piano Sonatas nos 1 and 29, some of his adaptations of Hardanger Folk-Songs and also the Fourth Concerto for Piano and Orchestra – Aurora Borealis. The piano concerto was performed in a two-piano version, in which Tveitt was assisted by the French pianist Genevieve Joy. Tveitt's masterful playing, earned him the acclaim of his former teacher Nadia Boulanger in her following review.

In spite of Tveitt's glorious successes internationally, the contemporary Norwegian establishment never recognized. Following the upheaval of World War II, anything that resembled nationalism or purism was quickly disdained by the establishment. Because of this Tveitt struggled financially and became increasingly isolated, spending more and more time at the family farm in Kvam and keeping his music to himself. His condition was worsened when his house burned to the ground in 1970. Tveitt despaired – the original manuscripts to almost 300 opuses (including six piano concertos and two concertos for Hardanger fiddle and orchestra) were destroyed in the fire. The Norwegian Music Information Centre agreed to archive the remains, but most of Tveitt's production was gone. Tveitt now found it very difficult to compose and gradually fell into alcoholism. Tveitt died in Norheimsund, Hardanger, largely embittered, with little hope for the legacy of his professional work.

One of the most controversial areas of Tveitt's career is his affiliation with the Neo-Heathenistic movement, which centered around the Norwegian philosopher Hans S. Jacobsen (1901–1980) in the 1930s in Oslo. Jacobsen's main thesis, inspired by the theories of the German theologist Jakob Wilhelm Hauer, was the total refutation of Christianity in favour of a new heathen system based upon Norse mythology and the Edda poetry. The movement rejected Christianity and sought to re-introduce the Norse pre-Christian system of belief – the adoration of Odin, Thor and Balder. Jacobsen later became a member of Nasjonal Samling ('National Assembly'), which led the interim, pro-Hitler puppet government during the German occupation of Norway. Even though Geirr Tveitt displayed a deep interest in the theories of the movement, he never enrolled as a member of the Nasjonal Samling. His preoccupation with Jacobsen's thinking however, materialised in conspicuous ways; for example, Tveitt invented his own non-Christian timeline based upon the arrival of Leif Erikson in what is now Canada. Traces of Antisemitism are often found in his correspondence from the 1930s. The Neo-Heathen system of thought found its way into Tveitt's music; his perhaps most intensely such composition is the ballet Baldur's Dreams. In it Tveitt seeks to establish a link between this world – its creation, cycle and dwellers – and the eternal battle between the benevolent heathen Norse gods and their opponents, the evil jotuns. Tveitt began work on the ballet whilst studying in Leipzig, where it was first performed on 24 February 1938. There Baldur's Dreams became a remarkable success, and performances were later given in Berlin, Tübingen, Bergen and Oslo.

Another result of Tveitt's Norse purism was his development of the theory that the modal scales originally were Norwegian, renaming them in honor of Norse gods. He also developed an intricate diatonic theory, which interconnected the modal scales through a system of double leading notes.^{[citation needed]} These ideas were published in his 1937 argument Tonalitätstheorie des parallellen Leittonsystems.