The Civil Rights Act of 1866 (14 Stat. 27–30, enacted April 9, 1866, reenacted 1870) was the first United States federal law to define citizenship and affirm that all citizens are equally protected by the law. It was mainly intended, in the wake of the American Civil War, to protect the civil rights of persons of African descent born in or brought to the United States.

The Act was passed by Congress in 1866 and vetoed by U.S. President Andrew Johnson. In April 1866, Congress again passed the bill to support the Thirteenth Amendment, and Johnson again vetoed it, but a two-thirds majority in each chamber overrode the veto to allow it to become law without presidential signature.

John Bingham and other congressmen argued that Congress did not yet have sufficient constitutional power to enact this law. Following passage of the Fourteenth Amendment in 1868, Congress ratified the 1866 Act in 1870.

The act had three primary objectives for the integration of African Americans into the American society following the Civil War: 1.) a definition of American citizenship 2.) the rights which come with this citizenship and 3.) the unlawfulness to deprive any person of citizenship rights "on the basis of race, color, or prior condition of slavery or involuntary servitude." The act accomplished these three primary objectives.

The author of the Civil Rights Act of 1866 was United States Senator Lyman Trumbull. Congressman James F. Wilson summarized what he considered to be the purpose of the act as follows, when he introduced the legislation in the House of Representatives:

It provides for the equality of citizens of the United States in the enjoyment of "civil rights and immunities." What do these terms mean? Do they mean that in all things civil, social, political, all citizens, without distinction of race or color, shall be equal? By no means can they be so construed. Do they mean that all citizens shall vote in the several States? No; for suffrage is a political right which has been left under the control of the several States, subject to the action of Congress only when it becomes necessary to enforce the guarantee of a republican form of government. Nor do they mean that all citizens shall sit on the juries, or that their children shall attend the same schools. The definition given to the term "civil rights" in Bouvier's Law Dictionary is very concise, and is supported by the best authority. It is this: "Civil rights are those which have no relation to the establishment, support, or management of government."

During the subsequent legislative process, the following key provision was deleted: "there shall be no discrimination in civil rights or immunities among the inhabitants of any State or Territory of the United States on account of race, color, or previous condition of servitude." John Bingham was an influential supporter of this deletion, on the ground that courts might construe the term "civil rights" more broadly than people like Wilson intended. Weeks later, Senator Trumbull described the bill's intended scope:

This bill in no manner interferes with the municipal regulations of any State which protects all alike in their rights of person and property. It could have no operation in Massachusetts, New York, Illinois, or most of the States of the Union.