The City of Unalaska (Aleut: Iluulux̂; Russian: Уналашка) is the main population center in the Aleutian Islands. The city is in the Aleutians West Census Area, a regional component of the Unorganized Borough in the U.S. state of Alaska. Unalaska is located on Unalaska Island and neighboring Amaknak Island in the Aleutian Islands off mainland Alaska. The population was 4,254 at the 2020 census, which is 81% of the entire Aleutians West Census Area. Unalaska is the second-largest city in the Unorganized Borough, behind Bethel.

The Aleut (Unangan) people have lived on Unalaska Island for thousands of years. The Unangan, who were the first to inhabit the island of Unalaska, named it "Ounalashka", meaning "near the peninsula". The regional native corporation has adopted this moniker, and is known as the Ounalashka Corporation. The Russian fur trade reached Unalaska when Stepan Glotov and his crew arrived on August 1, 1759. Natives, Russians and their Alaskan Creole descendants comprised most of the community's population until the mid-20th century, when the involvement of the United States in World War II led to a large-scale influx of people and construction of buildings all along the strategically located Aleutians.

Almost all of the community's port facilities are on Amaknak Island, better known as Dutch Harbor or just "Dutch". It is the largest fisheries port in the U.S. by volume caught. It includes Dutch Harbor Naval Operating Base and Fort Mears, U.S. Army, a U.S. National Historic Landmark. Dutch Harbor lies within the city limits of Unalaska and is connected to Unalaska by a bridge. Amaknak Island is home to almost 59 percent of the city's population, although it has less than 3 percent of its land area.

As a result of the town's strong fishing industry, Unalaska has also become notable for its large population of bald eagles, which number in the hundreds and were attracted to the area by human activity.

The island of Unalaska was first inhabited by the Aleut people.

Unalaska and Amaknak islands contained 24 settlements with more than 1,000 Aleut inhabitants in 1759, when the first Russian group under Stepan Glotov came and started trading for three years on Umnak and Unalaska. Between 1763 and 1766, a conflict between the Russian fur traders and the Unalaska Natives occurred; the Aleuts destroyed four Russian ships and killed 175 hunters/traders. In the 1760s, Unalaska was temporarily used as a Russian fur trading post. The post was permanently established in 1774, and was eventually incorporated into the Russian-American Company. It was there that Captain James Cook encountered the navigator Gerasim Izmailov in 1778.

In 1788, the Spanish made contact with the Russians in Alaska for the first time. An expedition by Esteban José Martínez and Gonzalo López de Haro visited several Russian settlements. Their westernmost visit was to Unalaska. On August 5, 1788, they claimed Unalaska for Spain, calling it Puerto de Dona Marie Luisa Teresa.

Alexander Andreyevich Baranov was shipwrecked here in 1790.