The parish of Christianssund was established as a municipality on 1 January 1838 (see formannskapsdistrikt law). Initially, the small island municipality included just the town of Christianssund and its immediate surrounding area. During the 1960s, there were many municipal mergers across Norway due to the work of the Schei Committee. On 1 January 1964, Kristiansund Municipality was merged with the tiny Grip Municipality (population: 104) to the northwest and the Dale area of Bremsnes Municipality on Nordlandet island (population: 963). The neighboring Frei Municipality was merged with Kristiansund on 1 January 2008 creating a much larger Kristiansund Municipality.
The municipality is named after the town of Kristiansund. Historically, it was spelled Christianssund. The name comes from the Danish-Norwegian King Christian VI who founded the town in 1742. The last element of the name, sund, means "strait". The old name of the town/village (originally the island Kirkelandet) was Fosna or Fosen (Old Norse: fólgsn) which means "hiding place" (here 'hidden port'). It was also often named Lille Fosen ("the small Fosen") to distinguish it from the island Storfosna ("the big Fosen") in Ørland.
Before 1877, the name was spelled Christianssund, from 1877 to 1888 it was spelled Kristianssund, and since 1889 it has had its present spelling, Kristiansund.
Before the introduction of postal codes in Norway in 1968, it was easy to confuse the name Kristiansund with Kristiansand in the south. It was therefore obligatory to always add an N (for north) to Kristiansund (Kristiansund N) and an S (for south) to Kristiansand (Kristiansand S). This is pretty much still practiced and also occurs in some other contexts than postal addresses.
Coat of arms
The coat of arms was granted on 27 June 1742. The arms were granted by King Christian VI and are described as a silver or white river flowing from a cliff, with salmon jumping upwards on a blue background. The waterfall may possibly be the Lille Fosen waterfall near the town.
There are two myths as to why the arms show a waterfall. The first one is because the old name of the town (Fosen) was misinterpreted as Fossund (as a compound of foss which means waterfall and sund which means strait).
The other myth concerning the coat of arms is that there was a mix up, between Kristiansund's and Molde's intended shield. The Dano-Norwegian government officials in charge of the giving of the coats, had a party to remember the momentous occasion and became too drunk and hungover to remember which was which, and so Molde got the coat with a whale (which are scarce in between the Romsdal fjords) and Kristiansund got the waterfall (since Molde is on the mainland and Kristiansund lies in the open sea, it would be more likely that the waterfall was intended for Molde's mountains and the whales for Kristiansund.)
The Church of Norway has three parishes (sokn) within the municipality of Kristiansund. It is part of the Ytre Nordmøre prosti (deanery) in the Diocese of Møre.
St. Eystein Catholic Church is the only Catholic church in Kristiansund.