The largest one-day percentage fall in the Dow Jones Industrial Average during a 20th-century crash was 22.6%, recorded on October 19, 1987.
That Monday became known as Black Monday. The Dow dropped 508 points, which represented 22.6% of its previous closing value. Because the index was much higher than in earlier decades, the point loss was also historically large, but the percentage figure is the standard way economists compare market collapses across eras.
The decline was part of a global sell-off. Markets in Asia and Europe fell before Wall Street opened, and selling accelerated in New York. Concerns included high valuations, trade tensions, interest rates, and automated portfolio-insurance strategies that encouraged further selling as prices dropped.
The crash did not lead to a depression comparable to the 1930s. The Dow later recovered, and regulators introduced measures such as circuit breakers to pause trading during extreme movements. Black Monday is therefore remembered both as a record market shock and as a catalyst for changes in market infrastructure.