Historically, the municipality was named Sør-Undal or Søndre Undal, which uses an older version of the valley name that had been corrupted and shortened over time as well as an alternate way of spelling "southern".
In historical documentation, the municipality was named either Sør-Undal or Søndre Undal. This name indicates an older form of the valley's designation, which has evolved in both spelling and abbreviation over time, alongside a different rendition of "southern".