1866 in the United States

Decades:	1840s; 1850s; 1860s; 1870s; 1880s;
See also:	; History of the United States (1865–1918); Timeline of United States history (1860–1899); List of years in the United States;

Events from the year 1866 in the United States.

Incumbents

January – The second and current United States Capitol dome is completed after 11 years of work.
February 13 – The first daylight bank robbery in United States history during peacetime takes place in Liberty, Missouri. This is considered to be the first robbery committed by Jesse James and his gang, although James's role is disputed.

April 9 – The Civil Rights Act of 1866 is passed, overriding a presidential veto, granting further rights to freed slaves.
April 10 – The American Society for the Prevention of Cruelty to Animals (ASPCA) is founded in New York City by Henry Bergh.
May 16 – The U.S. Congress approves the minting of a nickel 5-cent coin (nickel), eliminating its predecessor, the half dime.

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July 4 – The Great Fire of Portland, Maine kills two and leaves 10,000 homeless in the worst fire in an American city at the time.
July 23 – The Judicial Circuits Act reduces the number of United States circuit courts to nine and the number of Supreme Court justices to seven.
July 24 – Reconstruction: Tennessee becomes the first U.S. state to be readmitted to the Union following the American Civil War.
July 25 – The U.S. Congress passes legislation authorizing the rank of General of the Army (now called "5-star general"); Lieutenant General Ulysses S. Grant becomes the first to have this rank.
July 28 – The Metric Act of 1866 becomes law and legalizes the use of the metric system for weights and measures in the United States.
August 14–16 – The National Union Convention is held in Philadelphia with hopes to reconcile the Radical Republicans in Congress with the Reconstructionist policies of President Andrew Johnson.
August 27–September 15 – President Andrew Johnson goes on his Swing Around the Circle speaking tour to gain support for his Reconstructionist policies and Democratic Party candidates in the upcoming elections.

October 7–21 – The Second Plenary Council of American Roman Catholic bishops is held in Baltimore.
November 5 – House of Representatives elections: Despite President Andrew Johnson's Swing Around the Circle tour, the Republican Party wins in a landslide.