Vefsn (Southern Sami: Vaapste) is a municipality in Nordland county, Norway. It is part of the Helgeland traditional region. The administrative centre of the municipality is the town of Mosjøen (population: 9,843). Some of the notable villages in Vefsn include Drevvassbygda, Elsfjord, and Husvik.

The 1,929-square-kilometre (745 sq mi) municipality is the 41st largest by area out of the 357 municipalities in Norway. Vefsn Municipality is the 91st most populous municipality in Norway with a population of 13,469. The municipality's population density is 7 inhabitants per square kilometre (18/sq mi) and its population has increased by 1.4% over the previous 10-year period.

The municipality of Vefsn was established on 1 January 1838 (see formannskapsdistrikt law). In 1862, the vast eastern district (population: 921) was separated from Vefsn Municipality to become the new Hattfjelldal Municipality. This left Vefsn Municipality with 5,051 residents. In 1876, the town of Mosjøen (population: 379) was separated from Vefsn Municipality to become a separate town-municipality. This left Vefsn Municipality with 4,672 residents.

In 1927, Vefsn Municipality was split into three: the small northern district (population: 964) became the new Drevja Municipality and the large southern district (population: 1,746) became the new Grane Municipality. After the split, Vefsn Municipality had 3,119 residents remaining. In 1939, a small area of Vefsn Municipality (population: 45) was transferred to neighboring town of Mosjøen.

During the 1960s, there were many municipal mergers across Norway due to the work of the Schei Committee. On 1 January 1962, Vefsn Municipality (population: 5,358) was merged with the neighboring town of Mosjøen (population: 4,628) and the neighboring Drevja Municipality (population: 1,001) and Elsfjord Municipality (population: 920) to form a new, larger Vefsn Municipality. On 1 January 1995, the mainland areas of the neighboring Alstahaug Municipality (population: 70) were transferred to Vefsn Municipality.

The municipality (originally the parish) is named after the river Vefsna (Old Norse: Vefsnar) which flows through the municipality into the Vefsnfjorden (Old Norse: Vefsnir). The meaning of the river name is unknown, but it possibly comes from the old word vefja which means "to wrap" or "to wind around". Historically, the name of the municipality was spelled Vefsen. On 3 November 1917, a royal resolution changed the spelling of the name of the municipality to Vefsn.

The coat of arms was granted on 13 September 1974. The official blazon is "Sable, a rooster argent armed gules" (Norwegian: På svart bunn en sølv hane). This means the arms have a black field (background) and the charge is a rooster. The rooster has a tincture of argent which means it is commonly colored white, but if it is made out of metal, then silver is used. The rooster is also armed which means its claws, beak, and comb are colored red. The rooster is a symbol for watchfulness and alertness. The arms were designed by Arthur Gustavsson.

The coat of arms was originally adopted by the municipality of Mosjøen on 25 March 1960. In 1962, Mosjøen was merged with Vefsn Municipality, Drevja Municipality, and Elsfjord Municipality to form a new, larger Vefsn Municipality. After the merger, the old arms of Mosjøen were chosen for the new municipality. Since the arms officially became obsolete after the merger, they were re-granted in 1974.