Hailuoto (Swedish: Karlö) is a Finnish municipality located mostly on an island of the same name in the Bay of Bothnia in the province of Northern Ostrobothnia. The unique landscape and natural conditions have made Hailuoto, the largest island in the Gulf of Bothnia, a national landscape. Of all the islands in Finland's sea areas, Hailuoto is the third largest after Åland and Kemiönsaari. The first parts of the island are estimated to have risen from the sea about 1,700 years ago, and it has taken its current form as an association of several different islands. As the uprising continues, it will eventually grow onto the mainland.

Hailuoto has a population of 959 and is the smallest municipality in the province of Northern Ostrobothnia and the former county of Oulu. The neighboring municipalities of Hailuoto are Lumijoki, Oulu and Siikajoki. The former neighboring municipalities are Haukipudas and Oulunsalo, which are connected to Oulu.

Hailuoto is not only the name of the municipality (administrative area), but also the name of the island (geographical area). The island of Hailuoto is located in the Gulf of Bothnia off Oulu. It is the only permanently inhabited island in the Bay of Bothnia with no fixed road connection.

Marjaniemi lighthouse is located in the westernmost part of Hailuoto, Marjaniemi. The former pilot station building of the Pilotage Enterprise, located at the foot of the lighthouse, now houses the Pilot Hotel, and the Bay of Bothnia Research Station of the University of Oulu has operated in the building next to the lighthouse.

The whole of Hailuoto is classified as one of Finland's nationally significant built cultural environments.

In the 1980s, fish sticks made from whitefish were named Hailuoto's parish food.