Lyndon Hermyle LaRouche Jr. (September 8, 1922 – February 12, 2019) was an American political activist who founded the LaRouche movement and its main organization, the National Caucus of Labor Committees (NCLC). He was a prominent conspiracy theorist and perennial presidential candidate. He began in far-left politics in the 1940s and later supported the civil rights movement; however, in the 1970s, he moved to the far-right. His movement is sometimes described as, or likened to, a cult. Convicted of fraud, he served five years in prison from 1989 to 1994.

Born in Rochester, New Hampshire, LaRouche was drawn to socialist and Marxist movements in his twenties during World War II. In the 1950s, while a Trotskyist, he was also a management consultant in New York City. By the 1960s, he became engaged in increasingly smaller and more radical splinter groups. During the 1970s, he created the foundation of the LaRouche movement and became more engaged in conspiratorial beliefs and violent and illegal activities. Instead of the radical left, he embraced radical right politics and antisemitism. At various times, he alleged that he had been targeted for assassination by Queen Elizabeth II, Zionist mobsters, his own associates (whom he said had been drugged and brainwashed by CIA and British spies), in addition to others.

It is estimated that the LaRouche movement never exceeded a few thousand members, but it had an outsized political influence, raising more than $200 million by one estimate, and running candidates in more than 4,000 elections in the 1980s. It was noted for disguising its candidates as conservative Democrats and harassing opponents. It reached its height in electoral success when Larouchite candidates won the Democratic primaries for the 1986 Illinois gubernatorial election and related state offices; this alarmed Democratic Party officials, whose national spokesman called the Larouchites "kook fringe". The defeated mainstream Democratic candidates ran in the general election as members of the Illinois Solidarity Party; the Larouchite Democrats all finished a distant third. Later in the 1980s, as part of the LaRouche criminal trials, criminal investigations led to convictions of several LaRouche movement members, including LaRouche himself. He was sentenced to 15 years' imprisonment but served only five.

LaRouche was a perennial candidate for President of the United States. He ran in every election from 1976 to 2004 as a candidate of third parties established by members of his movement, peaking at around 78,000 votes in the 1984 United States presidential election. He also tried to gain the Democratic presidential nomination. In the 1996 Democratic Party presidential primaries, he received 5% of the total nationwide vote. In 2000, he received enough primary votes to qualify for delegates in some states, but the Democratic National Committee refused to seat his delegates and barred LaRouche from attending the Democratic National Convention.

LaRouche was born in Rochester, New Hampshire, the oldest of three children of Jessie Lenore (née Weir) and Lyndon H. LaRouche Sr. His paternal grandfather's family emigrated to the United States from Rimouski, Quebec, whereas his maternal grandfather was born in Scotland. His father worked for the United Shoe Machinery Corporation in Rochester before the family moved to Lynn, Massachusetts.

His parents became Quakers after his father converted from Catholicism. They forbade him from fighting with other children, even in self-defense, which he said led to "years of hell" from bullies at school. As a result, he spent much of his time alone, taking long walks through the woods and identifying in his mind with great philosophers. He wrote that, between the ages of 12 and 14, he read philosophy extensively, embracing the ideas of Leibniz and rejecting those of Hume, Bacon, Hobbes, Locke, Berkeley, Rousseau, and Kant. He graduated from Lynn English High School in 1940. In the same year, the Lynn Quakers expelled his father from the group, for reportedly accusing other Quakers of misusing funds, while writing under the pen name Hezekiah Micajah Jones. LaRouche and his mother resigned in sympathy with his father.

LaRouche attended Northeastern University in Boston and left in 1942. He later wrote that his teachers "lacked the competence to teach me on conditions I was willing to tolerate". As a Quaker, he was a conscientious objector during World War II and joined a Civilian Public Service camp in lieu of military service. In 1944, he decided to enlist in the United States Army and served with the Medical Corps in India and Burma during the Burma campaign. At the end of the war, he was working as a clerk in the Ordnance Corps, and later described his decision to enlist as the most important decision of his life. In his 1988 autobiography, LaRouche said that being asked to express his views on the death of President Franklin D. Roosevelt to a group of fellow G.I.s led him to define his "principal lifelong political commitment, that the United States should take postwar world leadership in establishing a world order dedicated to promoting the economic development of what we call today "developing nations".

LaRouche wrote that, while in the conscientious objector camp, he discussed Marxism, and while traveling home on the SS General Bradley in 1946, he met Donald Merrill, a fellow soldier, also from Lynn, who converted him to Trotskyism. Back in the U.S., he resumed his education at Northeastern University but dropped out. He returned to Lynn in 1948 and the next year joined the Socialist Workers Party (SWP) to recruit at the GE River Works there, adopting the name "Lyn Marcus" for his political work. He arrived in New York City in 1953, where he worked as a management consultant. In 1954 he married Janice Neuberger, a member of the SWP. Their son, Daniel, was born in 1956.