Oliver Wendell Holmes Jr. (March 8, 1841 – March 6, 1935) was an American jurist who served as an associate justice of the U.S. Supreme Court from 1902 to 1932. Holmes is one of the most widely cited and influential Supreme Court justices in American history, noted for his long tenure on the Court and for his pithy opinions – particularly those on civil liberties and American constitutional democracy – and deference to the decisions of elected legislatures. Holmes retired from the Court at the age of 90, an unbeaten record for oldest justice on the Supreme Court. He previously served the Union as a brevet colonel in the American Civil War (in which he was wounded three times), as an associate justice and chief justice of the Massachusetts Supreme Judicial Court, and as Weld Professor of Law at his alma mater, Harvard Law School. His positions, distinctive personality, and writing style made him a popular figure, especially with American progressives.

During his tenure on the U.S. Supreme Court, to which he was appointed by President Theodore Roosevelt in 1902, he supported the constitutionality of state economic regulation and came to advocate broad freedom of speech under the First Amendment, after, in Schenck v. United States (1919), having upheld for a unanimous court criminal sanctions against draft protestors with the memorable maxim that "free speech would not protect a man in falsely shouting fire in a theatre and causing a panic" and formulating the groundbreaking "clear and present danger" test. Later that same year, in his famous dissent in Abrams v. United States (1919), he wrote that "the best test of truth is the power of the thought to get itself accepted in the competition of the market. ... That, at any rate, is the theory of our Constitution. It is an experiment, as all life is an experiment." He added that "we should be eternally vigilant against attempts to check the expression of opinions that we loathe and believe to be fraught with death...."

The Journal of Legal Studies has identified Holmes as the third-most-cited American legal scholar of the 20th century. Holmes was a legal realist, as summed up in his maxim, "The life of the law has not been logic: it has been experience". He was also a moral skeptic and an opponent of the doctrine of natural law. His jurisprudence and academic writing influenced much subsequent American legal thinking, including the judicial consensus upholding New Deal regulatory law, "sociological jurisprudence in the early twentieth century, and ... much of Legal Realism a generation later".

Holmes was born in Boston, Massachusetts, to the prominent writer and physician Oliver Wendell Holmes Sr. and Amelia Lee Jackson Holmes. Both his parents were of English descent, and all his ancestors had come to North America from England during the early colonial period as part of the Puritan migration to New England. His mother opposed slavery and fulfilled her domestic role as traditionally understood. Dr. Holmes was a leading figure in Boston intellectual and literary circles. Mrs. Holmes was connected to the leading families; Henry James Sr., Ralph Waldo Emerson, and other transcendentalists were family friends.

Known as "Wendell" in his youth, Holmes became lifelong friends with the brothers William James and Henry James Jr. Holmes accordingly grew up in an atmosphere of intellectual achievement and early on formed the ambition to be a man of letters like Emerson. He retained an interest in writing poetry throughout his life. While still in Harvard College he wrote essays on philosophic themes and asked Emerson to read his attack on Plato's idealist philosophy. Emerson famously replied, "If you strike at a king, you must kill him." During his years as a Harvard undergraduate, Holmes was a devoted abolitionist and once served as a bodyguard for Wendell Phillips, the radical abolitionist leader. He was also a member of Alpha Delta Phi fraternity, the Hasty Pudding and the Porcellian Club; his father had also been a member of both clubs. In the Pudding, he served as Secretary and Poet, as had his father. Holmes graduated Phi Beta Kappa and cum laude from Harvard in 1861, and, in the spring of that year, after President Abraham Lincoln called for volunteers following the firing on Fort Sumter, he enlisted in the Massachusetts militia but returned briefly to Harvard College to participate in commencement exercises.

During his senior year of college, at the outset of the American Civil War, Holmes enlisted in the Fourth Battalion of Infantry in the Massachusetts militia. Then, in July 1861, with his father's help, he received a commission as second lieutenant in the Twentieth Regiment of Massachusetts Volunteer Infantry. He saw considerable combat, taking part in the Peninsula Campaign and the Battle of the Wilderness, being wounded at the Battle of Ball's Bluff, Antietam, and Chancellorsville, and suffering from a near-fatal case of dysentery. He particularly admired and was close to Henry Livermore Abbott, a fellow officer in the 20th Massachusetts. Holmes rose to the rank of lieutenant colonel, but eschewed command of his regiment upon his promotion. Abbott took command of the regiment in his place and was later killed.

In September 1863, while recovering at the Holmes family home on Charles Street in Boston from his third major combat injury, Holmes was promoted to colonel, but he never returned to the 20th Massachusetts because the unit had been largely destroyed. Upon his recovery, in January 1864 Holmes was appointed aide-de-camp of General Horatio Wright, then Division Commander of VI Corps, and later in command of the Corps. Holmes served with Wright during General Grant's campaign down to Petersburg, returning to Washington with the Sixth Corps when the Capital was threatened in July 1864. On July 17, 1864, Holmes was mustered out at the end of his enlistment term, returning to Boston and enrolling at Harvard Law School later that year.

Holmes is said to have shouted to Abraham Lincoln to take cover during the Battle of Fort Stevens, although this is commonly regarded as apocryphal. Holmes himself expressed uncertainty about who had warned Lincoln ("Some say it was an enlisted man who shouted at Lincoln; others suggest it was General Wright who brusquely ordered Lincoln to safety. But for a certainty, the 6 foot 4 inch Lincoln, in frock coat and top hat, stood peering through field glasses from behind a parapet at the onrushing rebels.") and other sources state he likely was not present on the day Lincoln visited Fort Stevens.