Louis Dembitz Brandeis (November 13, 1856 - October 3, 1941) was an important American litigator, Justice, advocate of privacy, and developer of the Brandeis Brief. In addition, he helped lead the American Zionist movement. He was appointed by Woodrow Wilson to the Supreme Court of the United States in 1916 (sworn-in on June 5), and served until 1939. He was the first Jew to hold that office. Before his appointment to the Supreme Court, he was associated with the progressive wing of the United States Democratic Party, and published a notable book in support of competition rather than monopoly in business.

Brandeis was born in Louisville, Kentucky, the scion of a Jewish family originating from Prague.

In the 1907 case Muller v. Oregon, Brandeis collected empirical data from hundreds of sources. In what became known as the "Brandeis Brief", the report provided social authorities on the issue of impact of long working hours on women. This was the first instance in the United States that social science had been used in law and changed the direction of the Supreme Court and of U.S. law. The Brandeis Brief became the model for future Supreme Court presentations.

Overcoming significant opposition to his appointment, Brandeis became one of the most influential and respected Supreme Court Justices in United States history. His votes and opinions envisioned the greater protections for individual rights and greater flexibility for government in economic regulation that would prevail in later courts. In his widely-cited dissenting opinion in Olmstead v. United States (1928), Brandeis argued, as he had in an influential law review article prior to being nominated to the Court that the Constitution protected a "right of privacy," calling it "the most comprehensive of rights and the right most valued by civilized men." Brandeis' position in Olmstead became the law of the land in 1967's Katz v. United States, which overturned Olmstead. Additionally, Brandeis joined with fellow justice Oliver Wendell Holmes, Jr. in calling for greater Constitutional protection for speech, dissenting from the Court's upholding of a conviction for aiding the Communist Party in Whitney v. California (1927), a dissent that foreshadows the greater speech protections enforced by the Warren Court. Bradeis also vigorously opposed the Supreme Court's doctrine of "liberty of contract," which acted to shield business from government regulation on the basis of a fictional equality between employers and employees in making contracts. In New State Ice Co. v. Leibmann (1932), Brandeis in dissent famously urged that the states should be able to be "laboratories" for innovative government action, in the face of the Supreme Court's frequent invalidation of state measures regulating business. Here, too, Brandeis's views would prevail in the long run.

Brandeis University, in Waltham, Massachusetts, was named after Louis Brandeis, as was the Brandeis Award. The University of Louisville features the Louis D. Brandeis School of Law, also named after him.

The remains of both Justice Brandeis and his wife are interred beneath the Louis D. Brandeis School of Law at the University of Louisville.