The parish of Christianssund was established as a municipality on 1 January 1838 (see formannskapsdistrikt). On 1 January 1897, the Grip archipelago was separated from Kristiansund Municipality to become a separate municipality. 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 city, formerly named Christianssund, is named after the Danish-Norwegian king Christian VI 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 written 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.
The Coat-of-arms is from 1742. It shows a waterfall 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). There are, however, no waterfalls in the municipality.
Another 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 deanery in the Diocese of Møre.
St. Eystein Catholic Church is the only Catholic church in Kristiansund.