Karlsøy (Northern Sami: Gálssa suohkan) is a municipality in Troms county, Norway. The administrative centre of the municipality is the village of Hansnes. Karlsøy is also the name of a village on Karlsøya island in the municipality of Karlsøy. The village of Karlsøy had a population (2001) of 70.
Karlsøy was established as a municipality on 1 January 1838 (see formannskapsdistrikt). On 1 January 1867, the district of Ullsfjord (population: 862) was transferred from Karlsøy to the neighboring municipality of Lyngen. On 1 September 1886, Helgøy was separated from Karlsøy to become a municipality of its own. This left Karlsøy with 1,334 inhabitants. Then on 1 January 1964, the municipality of Helgøy was merged back into the municipality of Karlsøy. At the same Time, all of the mainland areas of Karlsøy (the northern part of the Lyngen peninsula, with 1,001 inhabitants) was transferred from Karlsøy to neighboring Lyngen municipality. After all the changes, the new Karlsøy had 3,414 residents. On 1 January 2008, the southern part of the island of Reinøya was transferred from Tromsø to Karlsøy.