Vadsø (help·info) (Čáhcesuolu in Northern Sami or Vesisaari in Kven/Finnish) is a city and municipality in Finnmark county, Norway. The city is the administrative centre of the municipality and the county of Finnmark.

Vadsø was established as a municipality on 1 January 1838 (see formannskapsdistrikt). The law required that all cities should be separated from their rural districts, but because of a low population and very few voters, this was impossible to carry out for the municipality of Vadsø in 1838. (See also Hammerfest and Vardø.)

The rural districts of Vadsø were separated from the city in three stages: Nesseby (in 1846), Sør-Varanger (in 1858), and Nord-Varanger (in 1894). The last one was, however, merged back into the city on 1 January 1964.

The Old Norse form of the name was Vatnsøy. The first element is the genitive case of vatn which means "water" and the last element is øy which means "island". Therefore the meaning of the name is "the island with drinking water".

The coat-of-arms is from modern times. They were granted on 20 February 1976. The arms show the head of a reindeer stag in silver on a red background. The reindeer is the main domestic animal in the municipality and thus of great economic importance.