Aaron Vail (1796–1878) was an American diplomat who served as chargé d'affaires in England and Spain in the 1830s and 1840s.
He was born in Lorient, France, where his father, Aaron Vail (1758-1813), a prominent businessman and merchant from New York was serving as U.S. consul and commercial agent. Vail's French mother brought the Vail family to the United States after the senior Aaron Vail's death, and they resided in Washington, D.C.
The younger Aaron Vail was educated in Washington and became a clerk in the Department of State. In 1831 Martin Van Buren selected Vail to be the secretary of the U.S. legation in London; when Van Buren's appointment as Minister was rejected by the United States Senate, Vail acted as chargé d'affaires, from April 4, 1832 until 1836.
Vail served as a Special Diplomatic Agent to Canada in 1838.
From May 20, 1840 to August 1, 1842, Vail served as chargé d'affaires in Spain, remaining in the post until the arrival of Washington Irving to serve as Minister. After returning to the United States, Vail served for several years as the State Department's chief clerk, its top non-political appointment. He later lived in New York City; while in retirement, he declined diplomatic posts offered by Franklin Pierce and James Buchanan. He later moved to Paris, and died in Pau in 1878.
Vail's brothers included: Eugene, a State Department employee; Edward, an officer in the United States Navy; and Jefferson, an officer in the United States Army.