James Fisk Jr. (April 1, 1835 – January 7, 1872), known variously as "Big Jim", "Diamond Jim", and "Jubilee Jim" – was an American stockbroker and corporate executive who has been referred to as one of the "robber barons" of the Gilded Age. Though Fisk was admired by the working class of New York and the Erie Railroad, he achieved much ill-fame for his role in Black Friday in 1869, where he and his partner Jay Gould befriended the unsuspecting President Ulysses S. Grant in an attempt to use the President's good name in a scheme to corner the gold market in New York City. On January 7, 1872, Fisk was assassinated in New York City by his former mistress's new lover, who was trying to blackmail him.

Fisk was born in the hamlet of Pownal, Vermont, in Bennington County in 1835. After a brief period in school, he ran away in 1850 and joined Van Amberg's Mammoth Circus & Menagerie. Later, he became a hotel waiter, and finally adopted the business of his father, a peddler.

Fisk applied what he learned in the circus to his peddling and grew his father's business. He then became a salesman for Jordan Marsh, a Boston dry goods firm.

A failure as a salesman, he was sent to Washington, D.C., in 1861 to sell textiles to the government. By his shrewd dealing in army contracts during the Civil War, and, by some accounts, cotton smuggling across enemy lines—in which he enlisted the help of his father—he accumulated considerable wealth, which he soon lost in speculation. At the end of the war, he made a fortune by learning of the end of the Siege of Petersburg which guaranteed Confederate defeat and sending an agent on a fast boat to London to short as many Confederate bonds as possible before the news arrived.

In 1864 Fisk became a stockbroker in New York City, and was employed by railroad robber baron Daniel Drew as a buyer. He aided Drew in the Erie War against Cornelius Vanderbilt for control of the Erie Railroad. This resulted in Fisk and Jay Gould becoming members of the Erie directorate, and subsequently, a well-planned raid netted Fisk and Gould control of the railroad. The association with Gould continued until Fisk's death.

Fisk and Gould carried financial buccaneering to extremes: their program included an open alliance with New York politician Boss Tweed, the wholesale bribery of legislatures, and the buying of judges. Their attempt to corner the gold market culminated in the fateful Black Friday of September 24, 1869. Though many investors were ruined, Fisk and Gould escaped both punishment and significant financial harm.

Fisk married Lucy Moore in 1854, when he was 19 and she was 15. Lucy was an orphan, raised by an uncle from Springfield, Massachusetts. She tolerated Fisk's many extramarital affairs, perhaps because she was happy living with her “childhood friend and inseparable companion,” Fanny Harrod, in Boston. Regardless, they remained close, with Fisk visiting her every few weeks and spending summers and vacations with her every chance he could.

In New York, Fisk had a relationship with Josie Mansfield. Mansfield was considered by some a voluptuous beauty by Victorian standards of female desirability. Fisk housed Mansfield in an apartment a few doors down from the Erie Railroad headquarters on West 23rd Street and had a covered passage built linking the back doors of the headquarters and her apartment building. Fisk's relationship with Mansfield scandalized New York society.