We do not have a very clear picture of how northern Norway was administered by the king in the oldest time, but it appears that "Hålogaland" originally constituted one "fylke" (county). Later, during the Middle Ages, a division into districts took place. In Helgeland these were Herøy and Rødøy. By the end of the Middle Ages, around 1500, came a reorganization so that Herøy district, or Alstahaug district as it was also called, was divided into quarters. Sørsem (Sør-Sømn) with its 73 farms made up one such area. The other quarters in the old Brønnøy parish (one of the three parishes in Helgeland) were Velfjorden, Vega, and Nordsem (Nord-Sømn). Later Bindalen was added.

The court registers show that the quarters concept went out of use around 1780. From then on the parish Brønnøy, of which Vik parish was a pan, was often used as the head parish.Executive committee laws came in 1837, and Brønnøy (Nord-Sømn), Vik (Sdr-Sømn), and Velfjorden became what we call a municipal unity. Velfjorden separated in 1875, while Vik became its own township according to the royal resolution of June 13, 1900, and also its own parish after the resolution of January 12, 1900. The municipal name Sømna was decided upon by the "innenriksdepartement" on November 13, 1940, and went into use December 1 of the same year. From 1964 to 1977 Sømna was a part of the larger community of Brønnøy. Sømna after 1977 is not identical with Sømna before 1964. Fourteen farms from the earlier Sømna belong to Brønnøy today (farm numbers 3-14, 17 and 18). When we use the term "det gamle Sømna" (the old Sømna), these farms are included. In spite of changing designations for the areas with which we are dealing, we have chosen to use the name which comes most naturally - Sømna.