Abraham Alfonse Albert Gallatin (January 29, 1761 – August 12, 1849) was a Swiss-American ethnologist, linguist, politician, founder of New York University, diplomat, and United States Secretary of the Treasury.

Gallatin was born in Geneva, Switzerland to a wealthy family, emigrating to Massachusetts in 1780. For a brief period, he attempted to set himself up in business, and for an even briefer time taught French at Harvard University, finally purchasing land in Fayette County, Pennsylvania and moving there in 1784. (His land was in Virginia when he bought it, but became part of Pennsylvania soon afterward.) The Friendship Hill National Historic Site, his home overlooking the Monongahela River, is maintained by the National Park Service.

Almost immediately, Gallatin became active in Pennsylvania politics; he was a member of the state constitutional convention in 1789, and was elected to the state legislature in 1790.

In 1793, Gallatin won election to the United States Senate. When the Third Congress opened on December 2, 1793, he took the oath of office, but, on that same day, nineteen Pennsylvania Federalists filed a protest with the Senate that Gallatin did not have the minimum nine years of citizenship required to be a senator. The petition was sent to committee, which duly reported that Gallatin had not been a citizen for the required period. Gallatin rebutted the committee report, noting his unbroken residence of thirteen years in the United States, his 1785 oath of allegiance to the Commonwealth of Virginia, his service in the Pennsylvania legislature, and his substantial property holdings in the United States. The report and Gallatin's rebuttal were sent to a second committee. This committee also reported that Gallatin should be removed. The matter then went before the full Senate where the Gallatin was removed in a party-line vote of 14–12.

Gallatin's brief stint in the Senate was not without consequence. Gallatin had proven to be an effective opponent of Alexander Hamilton's financial policies, and the election controversy added to his fame. The dispute itself had important ramifications. At the time, the Senate held closed sessions. However, with the American Revolution only a decade ended, the senators were leery of anything which might hint that they intended to establish an aristocracy, so they opened up their chamber for the first time for the debate over whether to unseat Gallatin. Soon thereafter, open sessions became standard procedure for the Senate.

Entering the House of Representatives in 1795, he served in the fourth through sixth Congresses, and went on to become majority leader. He was an important leader of the new Republican Party, and its chief spokesman on financial matters. He opposed the entire program of Alexander Hamilton, though when he came to power he found himself keeping all the main parts.

As party leader, Gallatin put a great deal of pressure on Treasury Secretary Oliver Wolcott Jr. to maintain fiscal responsibility. He also helped found the House Committee on Finance (which would evolve into the Ways and Means Committee) and often engineered withholding of finances by the House as a method of overriding executive actions to which he objected. Among these was the Quasi-War, of which he was a vociferous foe. His measures to withhold naval appropriations during this period were met with vehement animosity by the Federalists, who accused him of being a French spy. It was the opinion of Thomas Jefferson that the Alien and Sedition Acts were passed largely as a way to rein in Gallatin.

Gallatin is honored with a statue in front of the U.S. Treasury Building in Washington, D.C.When Jefferson became President, Gallatin was appointed Secretary of the Treasury. Gallatin served in that post for thirteen years, the longest term in history for that office. During the first part of his tenure, he made great progress in balancing the federal budget. The United States was able to make the Louisiana Purchase without a tax increase in large part due to Gallatin's efforts. Gallatin also involved himself in the planning of the Lewis and Clark Expedition, mapping out the area to be explored.

In 1812, the United States was financially unprepared for war. For example, the Republicans allowed the First Bank of the United States to expire in 1811, over Gallatin's objections. He had to ship $7 million to Europe to pay off its foreign stockholders just at a time money was needed for war. The heavy military expenditures for the War of 1812, and the decline in tariff revenue caused by the embargo and the British blockade, sent the budget into the red. In 1813, the Treasury had expenditures of $39 million and revenue of only $15 million. Despite anger from Congress, Gallatin was forced to reintroduce the Federalist taxes he had denounced in 1798, such as the taxes on whiskey and salt, as well as a direct tax on land and slaves. He succeeded in funding the deficit of $69 million by bond issues, and thereby paid the direct cost of the war, which amounted to $87 million. He later helped charter the Second Bank of the United States in 1816.

In 1813, President James Madison sent him as the U.S. representative to a Russian-brokered peace talk, which Britain ultimately refused, preferring direct negotiations. Gallatin then resigned as Secretary of the Treasury to head the U.S. delegation for these negotiations in France and was instrumental in the securing of the Treaty of Ghent, which brought the War of 1812 to a close.

At war's end, Gallatin, preferring to remain in France, was appointed U.S. Minister to that country and held that post for another seven years. He returned to America in 1823 and was selected by the Democratic-Republican Party as its vice-presidential candidate, but was dissuaded by Martin Van Buren from accepting. Gallatin was alarmed at the possibility Andrew Jackson might win; he saw Jackson as "an honest man and the idol of the worshippers of military glory, but from incapacity, military habits, and habitual disregard of laws and constitutional. provisions, altogether unfit for the office."

He returned home to Pennsylvania where he lived until 1826.

By 1826, there was much contention between the United States and Britain over claims to the Columbia River system on the Northwest coast. Gallatin put forward a claim in favor of American ownership, outlining what has been called the "principle of contiguity" in his statement called "The Land West of the Rockies." It states that lands adjacent to already settled territory can reasonably be claimed by the settled territory. This argument is an early version of the doctrine of America's "manifest destiny". This principle became the legal premise by which the United States was able to claim the lands to the west.

In 1826 and 1927, he served as minister to the Court of St. James (i.e., minister to Great Britain).

He then settled in New York City, where he helped found New York University in 1831, in order to offer university education to the lower and middle classes. He became president of the National Bank (which was later renamed Gallatin Bank). In 1849, Gallatin died in Astoria on Long Island; he is interred at Trinity Churchyard in New York City.

Throughout his public service career, Gallatin pursued an interest in Native American language and culture. He drew upon government contacts in his research, gathering information through Lewis Cass, a Secretary of War, explorer William Clark and Thomas McKenney of the Bureau of Indian Affairs. He developed a personal relationship with Cherokee tribal leader John Ridge, who provided him with information on the vocabulary and structure of the Cherokee language. Gallatin's research resulted in two published works: A Table of Indian Languages of the United States (1826) and Synopsis of the Indian Tribes of North America (1836). His research led him to conclude that the natives of North and South America were linguistically and culturally related, and that their common ancestors had migrated from Asia in prehistoric times.

In 1842, Gallatin joined with John Russell Bartlett to found the American Ethnological Society. Later research efforts include examination of selected Pueblo societies, the Akimel O'odham (Pima) peoples, and the Maricopa of the Southwest. In politics, Gallatin stood for assimilation of Native Americans into European based American society, encouraging federal efforts in education leading to assimilation and denying annuities for Native Americans displaced by western expansion.

Gallatin's portrait was on the front of the $500 United States Note issued in 1862-1863.

Gallatin's portrait was on the standard 1¼¢ stamp from 1967-73.

Gallatin County, Montana is named in his honor.

Gallatin County, Kentucky is named in his honor.

Gallatin, Tennessee is named in his honor.

A school district in Fayette County, Pennsylvania was named in his honor.

The U.S. Department of the Treasury's highest career service award is named the Albert Gallatin Award in his honor.