Hailuoto (Finnish: [ˈhɑi̯ˌluo̯to]; Swedish: Karlö) is a Finnish island in the northern Baltic Sea and a municipality in Northern Ostrobothnia region. The population of Hailuoto is 928 (June 30, 2025), making it the smallest municipality in Northern Ostrobothnia and the former Oulu Province in terms of population. The municipality covers an area of 205.65 km² (79.40 sq mi) (excluding sea areas) of which 1.70 km² (0.66 sq mi) is inland water (January 1, 2018). The population density is 4.51/km² (11.7/sq mi). Of all the Finnish sea islands, Hailuoto is the third largest after Fasta Åland and Kimitoön.

Hailuoto lies opposite the city of Oulu in the Gulf of Bothnia. The distance between Oulu and Hailuoto is 53 kilometres (33 mi), and the sea area between them is called Luodonselkä (literally "open water of the islet").

Land in the region is constantly rising due to post-glacial rebound. It is estimated that the first parts of Hailuoto appeared from the Baltic Sea about 1700 years ago. The current island of Hailuoto was formed from many smaller islands. Two large sections, Santonen and Hanhinen merged into the main island (Luoto) only about two centuries ago. The island is continuously expanding and eventually it will join with the mainland. Kirkkosalmi, a wetland region between Hanhinen and Luoto is renowned for being an important bird refuge where rare bird species are observed.

The theme of the coat of arms of Hailuoto describes the economy of the island municipality; the explanation of the coat of arms is "in a blue field with a silver rowlock." The coat of arms was designed by Teuvo-Pentti Pakkala and approved by the Hailuoto Municipal Council at its meeting on 28 April 1966. The Ministry of the Interior approved the use of the coat of arms on 19 December of the same year.

The permanent settlement of Hailuoto apparently began in the 12th century. Residents came from many directions, especially from Karelia. The Swedes, who competed for its fishing waters, gave the island the name Karelö (which was transformed into Karlö), which means freely translated "Karelian Island". The first permanent residents of Hailuoto were mostly from Western Finland, but some also came from Eastern Finland. In 1548, there were 43 houses on the island, and by 1570 the number of houses had grown to 60. Queen Christina of Sweden donated the entire island of Hailuoto in 1652 to Colonel Berndt Taube, from whose heirs it was returned to the crown in 1675.

Hailuoto first belonged to the high parish of Saloinen (nowadays part of Raahe), from which it separated into an independent parish in 1587. The church, built in 1610–1620, was for a long time the oldest wooden church in use in Finland until it burned down in 1968. The current church was completed in 1972.

During the Great Famine of 1695–1697, 88 inhabitants of Hailuoto died. During the Great Northern War (1713–1721), 200 Cossacks of the Tsardom of Russia killed about 800 people in Hailuoto with axes on 29 September 1714, on so called "Murder Friday" (Murhaperjantai). Many of the victims had sought protection on the island, as Hailuoto had a population of less than 300 at that time. The bloodshed done overnight was the greatest of the Great Northern War. According to tradition, the bells of Hailuoto Church were drowned in Lake Kirkonjärvi during the Great Northern War, where they were never found again.

Hailuoto is the largest island in the Gulf of Bothnia, consisting of four parts, Luoto, Santonen, Hanhinen and Syökari. They were still separated a couple of centuries ago until they joined together as a result of crustal rebound. The young surface forms of the island are shaped throughout by sea waves, currents, ice thrust and wind.