The Southern Sami name became co-equal with the Norwegian one by national resolution on 14 June 2019 — Aarborte on its own, Aarborten tjïelte when the word for municipality is attached. That resolution came at the end of a long argument. Until well into the 1700s these valleys were used chiefly by migratory Sami herders; permanent settlement began around Røssvatnet about 1700, reached Susendalen roughly a century later, and drew Norwegian farmers from the eastern valleys in the late 1800s. What followed for Sami children was decades of state schooling designed to make them Norwegian. In 1951 a boarding school for Southern Sami children, Gaske-Nøørjen Saemienskovle, opened here — which meant children living away from their families through the school year, but also meant their language had a classroom. In 1987 the cultural centre Sijti Jarnge followed, driven largely by Anna Jacobsen, born Anna Kappfjell in 1924, who translated the Gospel of Mark into Southern Sami in 1993, ran Sami-language programming at NRK, and received the King's Medal of Merit in silver before her death in 2004.

The Norwegian name is literal to the point of bluntness. A mountain nearby, Hattfjellet, has a flat top and steep sides and looks like a hat; the valley beneath it took the mountain's name; a farm in that valley, recorded as Hatfieldalen in 1723, gave its name to the parish and then to the municipality. Spelling it proved harder than describing it. A royal resolution in 1908 dropped a d. Another in 1917 dropped the definite ending. A third, in 1926, added a t, and Hattfjelldal has stayed put since. When the coat of arms was granted on 24 October 1986, the designer Arvid Sveen simply drew the mountain: a white field above a green one, divided by a line with a single raised block in it, the profile of a flat-topped summit visible for a very long way.

Røssvatnet is the second-largest lake in Norway by surface area, and it is that size because engineers made it so. Before regulation it covered about 190 square kilometres at 374 metres above the sea. The dam first flooded it in 1957, raising the surface nine metres to 383 and the area to roughly 219 square kilometres — enough to move it up a place in the national rankings. Its shores have been occupied since the Stone Age, and in one season of 1957 they were redrawn. The reshaping had administrative consequences too. On 1 January 1964, during Norway's great wave of municipal mergers, the 168 people living on the north side of the lake were transferred out of Hattfjelldal to neighbouring Hemnes Municipality.

Hattfjelldal Airfield, ENHT, sits beside the administrative centre with a 715-metre concrete runway at 210 metres elevation. It began in 1933 as a simple landing field. After taking the area in 1940 the Germans expanded it into a proper airfield with a wooden runway, then replaced that with concrete by 1943, using the field as a staging point for raids on Narvik. That concrete is still there and still unmodified, which makes this the only wartime airfield in Norway surviving in its original form. Today it belongs to a flying club, and part of the hangar is used as storage by Arbor, the particleboard plant that is the municipality's largest industrial employer — a runway built for bombers now sharing space with pallets of chipboard.

At 2,684 square kilometres Hattfjelldal is the twentieth largest of Norway's 357 municipalities and the 315th most populous, which works out at roughly half a person per square kilometre. The population has fallen by more than fourteen per cent in a decade. Nearly a fifth of those who remain work in agriculture or forestry, and this is Nordland's leading sheep-farming district. The highest ground is Kvigtinden, measured at 1,699.92 metres on the Grane boundary; Børgefjell National Park occupies the southern end; the Vefsna, Nordland's largest river system, runs through on its 163-kilometre journey to the sea at Mosjøen. Not every entry in the ledger is picturesque. In November 2007 Hattfjelldal was one of eight Norwegian municipalities caught in the Terra Securities scandal, having pledged expected future hydropower revenues against leveraged American structured products that nobody involved appears to have fully understood.