The Alaska Purchase (Russian: Продажа Аляски, romanized: Prodazha Alyaski, lit. 'Sale of Alaska') was the United States' acquisition of Alaska from the Russian Empire. Alaska was formally transferred to the United States on October 18, 1867, through a treaty ratified by the United States Senate.

Russia had established a presence in North America during the first half of the 18th century, but few Russians ever settled in Alaska. In the aftermath of the Crimean War, Russian Tsar Alexander II began exploring the possibility of selling Alaska, which would be difficult to defend in any future war from being conquered by Russia's archrival, the United Kingdom of Great Britain and Ireland. Following the end of the American Civil War, U.S. Secretary of State William Seward entered into negotiations with Russian minister Eduard de Stoeckl for the purchase of Alaska. Seward and Stoeckl agreed to a treaty on March 30, 1867, and the treaty was ratified by the United States Senate by a wide margin.

The purchase added 586,412 square miles (1,518,800 km2) of new territory to the United States for the cost of $7.2 million 1867 dollars. In modern terms, the cost was equivalent to $133 million in 2020 dollars or $0.37 per acre. Reactions to the purchase in the United States were mostly positive, as many believed possession of Alaska would serve as a base to expand American trade in Asia. Some opponents labeled the purchase as "Seward's Folly", or "Seward's Icebox", as they contended that the United States had acquired useless land. Nearly all Russian settlers left Alaska in the aftermath of the purchase; Alaska would remain sparsely populated until the Klondike Gold Rush began in 1896. Originally organized as the Department of Alaska, the area was renamed the District of Alaska (1884) and the Alaska Territory (1912) before becoming the modern State of Alaska in 1959.