Black Monday occurred on October 19, 1987, when the Dow Jones Industrial Average suffered its largest one-day percentage decline in U.S. stock-market history.
The Dow fell 508 points, or 22.6%, in a single trading session. The decline was exceptionally severe because it happened across several major markets, including Hong Kong, London, and the United States. Investors around the world were selling rapidly, and computer-driven trading strategies intensified the downward movement.
The crash followed a long period of rising share prices and growing concern about interest rates, international trade, and the value of the U.S. dollar. No single event fully explains the collapse. Afterward, regulators introduced or strengthened safeguards, including circuit breakers designed to pause trading during unusually sharp declines.
Black Monday is often confused with the Wall Street Crash of 1929, which unfolded over several days and helped precede the Great Depression. The 1987 crash was much larger by one-day percentage loss, but it did not produce a depression on the same scale.