[5][6] The oldest remains of a boat ever found in Norway (the 2,500-year-old Haugvikbåten) was discovered in a bog in Sømna.

[8] The municipality (originally the parish) is named after the old Vik farm (Old Norse: Vík) since the first Sømna Church was built there.

[9] On 13 November 1940, a royal resolution changed the spelling of the name of the municipality to Sømna starting on 1 December 1940.

The official blazon is "Gules, three trefoils argent in pall stems conjoined" (Norwegian: I rødt tre sølv kløverblad forent i trepass).

This means the arms have a red field (background) and the charge is a group of three clover leaves (trefoils).

The clover 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 municipality of Sømna is mostly made up of the southern part of a peninsula off the mainland of Norway and the surrounding islands.

Sømna consists mainly of a wide Strandflaten lowland (coastal brim), and is one of few municipalities in Northern Norway where the farmers still grow some grain.

The municipal council (Kommunestyre) of Sømna is made up of 17 representatives that are elected to four year terms.