The Dow Jones Industrial Average’s largest one-day percentage loss was 22.6%, recorded on October 19, 1987.
That day became known as Black Monday. The Dow dropped 508 points, and selling spread rapidly across global markets. Although the point loss may look smaller than later declines when viewed with modern index levels, the percentage fall remains the Dow’s record for a single trading day.
The crash occurred amid concerns about overvalued shares, interest rates, international trade disputes, and market mechanisms that encouraged automatic selling. Portfolio insurance and other computer-assisted strategies were widely discussed as factors that may have amplified the decline.
The figure is often confused with the Dow’s record point losses, which belong to later periods when the index stood at much higher numerical levels. Percentage comparisons are more useful across decades because they account for the index’s changing scale.