Lierne is a municipality in Trøndelag county, Norway. It is part of the Namdalen region, and it is the largest municipality by area in Trøndelag. The administrative centre of the municipality is the village of Sandvika. Other villages include Inderdal, Mebygda, and Tunnsjø senter. The municipality borders Sweden to the south and east. Most of Lierne Municipality lies on the Swedish side of the drainage divide between Norway and Sweden.

The 2,962-square-kilometre (1,144 sq mi) municipality is the 16th largest by area out of the 357 municipalities in Norway. Lierne Municipality is the 314th most populous municipality in Norway with a population of 1,301. The municipality's population density is 0.4 inhabitants per square kilometre (1.0/sq mi) and its population has decreased by 6.1% over the previous 10-year period.

The municipality of Lierne was established on 1 January 1874 when it was separated from the large Snaasen Municipality. Initially, the population of Lierne Municipality was 1,015. On 1 July 1915, it was divided into two: Nordli Municipality (population: 863) in the north and Sørli Municipality (population: 739) in the south. This division, however, was short-lived. During the 1960s, there were many municipal mergers across Norway due to the work of the Schei Committee. On 1 January 1964, Nordli Municipality and Sørli Municipality were merged back together again under the former name Lierne Municipality. After the merger, there were 2,045 residents in the new municipality.

On 1 January 2018, the municipality switched from the old Nord-Trøndelag county to the new Trøndelag county.

The municipality (originally the parish) is named after the old name for the region (Old Norse: Hlíð) which means "mountainside" (similar to the Norwegian word li). Historically, the prefix Finna- was often added to the name: Finnahlíð. This prefix is the plural genitive case of finnr which means "Sami person" (or Finn) because the district historically had a significant sami population. Later, the prefix was dropped and the definite plural form of li was used, Lierne. The li name lives on in many places in the municipality such as Nordli and Sørli.

The coat of arms was granted on 3 February 1984. The official blazon is "Azure, three grouses argent, two over one" (Norwegian: I blått tre stående sølv ryper, to over en). This means the arms have a COLOR field (background) and the charge is three Willow Ptarmigans (Lagopus lagopus), a type of local grouse. The ptarmigan design has a tincture of argent which means it is commonly colored white, but if it is made out of metal, then silver is used. This design was chosen because these birds are plentiful in the area. Historically, hunting ptarmigan was of great importance for the survival of the inhabitants during the winters. The bird was thus chosen as a symbol for the municipality. The arms were designed by Einar H. Skjervold.

The Church of Norway has two parishes (sokn) within Lierne Municipality. It is part of the Namdal prosti (deanery) in the Diocese of Nidaros.

Lierne National Park is located in the municipality, and Blåfjella-Skjækerfjella National Park is partly located in Lierne. Lierne Municipality has a significant population of brown bears. The highest point in the municipality is the 1,390.09-metre (4,560.7 ft) tall mountain Hestkjøltoppen.