Sanak Island (Aleut: Sanaĝax) is an island in the Fox Islands group of the Aleutian Islands in the U.S. state of Alaska. It is located at 54°25′47″N 162°42′32″W / 54.42972°N 162.70889°W / 54.42972; -162.70889. Although currently uninhabited, it was historically the home of the Sanak Aleutians, who presently own the island via the Sanak Corporation. The island is accessible by fishing vessel, taking about five hours to cross the roughly 40 mile distance from King Cove to Sanak.

Sanak Island is part of the eastern Aleutians off the southern side of the Alaska Peninsula, lying south of the Ikatan Peninsula area of Unimak Island. Sanak Island and Caton Island are the largest islands in the Sanak Islands subgroup of the Fox Islands. The island is mostly low-lying, treeless, and contains many marshes, reefs, and shoals off the shore. Its highest point is Sanak Peak, which rises to approximately 1,740 ft (530 m).

Sanak is surrounded by numerous bays and anchorages, including Company Harbor or Sanak Harbor on the northwest side, Pauloff or Pavlof Harbor on the north coast, Johnson Bay, Sandy Bay, Peterson Bay, Salmon Bay, and Finneys Bay.

Like many of the other Aleutian Islands, Sanak was inhabited by the Unangax̂/Aleut people for thousands of years. Archaeological surveys as part of the Sanak Biocomplexity Project have uncovered a roughly 7,000 year record of inhabitation spread over 100 ancient village sites show a long maritime economy centered on the sea. Archaeological remains from ancient dumpsites show a broad subsistence base that included seafood, marine mammals, and occasional terrestrial animals brought from the Alaska Peninsula.

Sanak's location placed it within a wider North Pacific cultural and trade network. Obsidian found on Sanak has been traced to Okmok volcano on Umnak Island, about 250 miles away, while other stone materials were transported from the Alaska Peninsula.

Russians first learned of Sanak Island in 1762, when the fur trader Stepan Glotov encountered a Sanak Aleut boy who had been taken prisoner by Kodiak Islanders. According to historical accounts, the boy was taken aboard Glotov's ship and told them of the sea otter grounds around the island. A Russian vessel first landed on Sanak in 1771. After an initially friendly encounter, the relationship between the Russians and the Sanak Aleutians deteriorated after the leader took hostages and became involved in conflict with the Sanak and Alaska Peninsula Aleuts. They left Sanak the following summer, after attacks, food shortages, deaths, and disease.

Captain James Cook's expedition passed the island in 1778, where the crew reportedly caught over 100 halibut in a four-hour period, leading Cook to call the islands the "Halibut Isles". During the same visit, the expedition encountered an Aleut man in a baidarka, whom Cook compared to Indigenous people he had seen earlier in Prince William Sound and Kodiak.

Gradually the Russians returned, and began trading and working with Sanak hunters to collect otter pelts. In 1828, the administrators of Russian America removed the island's population to Belkofski on the Alaska Peninsula in order to maintain access to the sea otter hunting grounds in surrounding waters.