Which US stock index suffered the largest one-day percentage loss during the 1987 global crash?

The Dow Jones Industrial Average suffered the largest one-day percentage loss during the 1987 global crash, falling 22.6% on October 19.

That session became known as Black Monday. The decline was the largest single-day percentage drop in the Dow’s history, exceeding the losses recorded during the 1929 crash. Markets in Hong Kong, Europe, Australia, and elsewhere also fell sharply.

Several factors contributed, including elevated valuations, rising interest rates, trade and currency tensions, and portfolio-insurance strategies that automatically sold futures as prices declined. Computerized trading amplified selling pressure, although no single cause explains the worldwide collapse.

The Dow recovered much of its loss in later weeks, and the crash did not produce a depression comparable to the 1930s. Regulators subsequently introduced or strengthened circuit breakers and other mechanisms intended to slow trading during extreme market moves.