Louis Dembitz Brandeis (; November 13, 1856 – October 5, 1941) was an American lawyer and associate justice on the Supreme Court of the United States from 1916 to 1939.

Starting in 1890, he helped develop the "right to privacy" concept by writing a Harvard Law Review article of that title, and was thereby credited by legal scholar Roscoe Pound as having accomplished "nothing less than adding a chapter to our law". He later published a book entitled Other People's Money and How the Bankers Use It, suggesting ways of curbing the power of large banks and money trusts. He fought against powerful corporations, monopolies, public corruption, and mass consumerism, all of which he felt were detrimental to American values and culture. He also became active in the Zionist movement, seeing it as a solution to antisemitism in Europe and Russia, while at the same time being a way to "revive the Jewish spirit."

When his family's finances became secure, he began devoting most of his time to public causes and was later dubbed the "People's Lawyer". He insisted on serving on cases without pay so that he would be free to address the wider issues involved. The Economist magazine calls him "A Robin Hood of the law." Among his notable early cases were actions fighting railroad monopolies, defending workplace and labor laws, helping create the Federal Reserve System, and presenting ideas for the new Federal Trade Commission. He achieved recognition by submitting a case brief, later called the "Brandeis Brief", which relied on expert testimony from people in other professions to support his case, thereby setting a new precedent in evidence presentation.

In 1916, President Woodrow Wilson nominated Brandeis to a seat on the Supreme Court of the United States. His nomination was bitterly contested, partly because, as Justice William O. Douglas later wrote, "Brandeis was a militant crusader for social justice whoever his opponent might be. He was dangerous not only because of his brilliance, his arithmetic, his courage. He was dangerous because he was incorruptible ... [and] the fears of the Establishment were greater because Brandeis was the first Jew to be named to the Court." On June 1, 1916, he was confirmed by the Senate by a vote of 47 to 22, to become one of the most famous and influential figures ever to serve on the high court. His opinions were, according to legal scholars, some of the "greatest defenses" of freedom of speech and the right to privacy ever written by a member of the Supreme Court. In 1919, he ruled that the Eugene V. Debs protests against American involvement in World War I violated the Espionage Act of 1917 in Debs v. United States, though later that year had an apparent change of heart in Abrams v. United States when he dissented with the majority opinion to express that political dissent was protected by the First Amendment. Some have criticized Brandeis for evading issues related to African-Americans, as he did not author a single opinion on any cases about race during his twenty-three year tenure, and consistently voted with the court majority including in support of racial segregation.