The fourth and final great continental expansion that produced the present-day continental proportions of the United States was the purchase of Alaska from Russia in 1867. From the late eighteenth century, both Russia and America had sought out the northwest coast of North America for furs, mostly of sea otters and seals; Americans also pushed the whaling industry into the north Pacific. After the United States acquired California in 1848, San Francisco became the center of trading companies and other enterprises extending their activities northward. The most striking of these expansionist ventures was Perry Macdonough Collins's project to extend a telegraph line across western Canada, Alaska, and the Bering Strait to Siberia, where it would connect with Russian and other European lines. (Unfortunately, it could not compete with Cyrus Field's transatlantic cable for investment funds.)

American public interest in acquiring Alaska had already been rumored occasionally. At the time of the Crimean War, some Americans were considering taking over Russian holdings in the Alaska panhandle to keep them out of British hands, but the British avoided this action by agreeing to neutralize Russian property there. Even before then, a few American expansionists, especially William H. Seward, a Whig senator from New York, were attracted to Alaska. Seward regarded it as the key to an expanded Pacific commercialism pointed toward the Far East, especially China, which had long been a magnet for expansionist American businesspeople. At this time, Russia was losing interest in its remote, unprofitable colony. Fearful that the United States or Britain would take it over without paying for it, the czar and his government decided on the sale to the Americans. As secretary of state, Seward eagerly received the Russian overtures, and he and the Russian minister to the United States drew up a treaty literally overnight. The American public and press were taken by surprise and Seward and his fellow expansionists had to put together an intensive lobbying campaign to sell the treaty to Congress. The United States paid $7.2 million to Russia, and historians feel that perhaps $200,000 of that went to congressmen in one way or another to buy their votes for the bill appropriating the purchase price. (Perhaps many would have voted for it anyway.) Not for the first time in American history, expansionism was temporarily discredited, and later efforts to purchase the Danish West Indies and the Dominican Republic were voted down amid accusations of scandal. Nevertheless, the United States promptly set up a government in Alaska, and American business promoters started to take over Russian enterprises.

After the Civil War, the United States was unable to extend is borders northward into Canada. During the late 1860s, annexationism raised its head in British Columbia and Manitoba and then fell back. In the first case, Seward tried to encourage the discontented British Columbians, and in the second case, a group of New York and Minnesota railroadmen and other business interests tried to exploit a local rebellion early in the administration of Ulysses S. Grant. At an earlier date either case might have led to further continental expansion, but at this time, all the Americans could achieve was an arbitral award of the San Juan Islands by the German emperor in 1871. (These islands, left in dispute by a vague passage in the Oregon Treaty of 1846, were strategically located in the channel just off Vancouver Island.)