Which stock market suffered the largest one-day percentage loss in major index history on Black Monday in 1987?

The Hong Kong Stock Exchange suffered the largest one-day percentage loss among major markets during Black Monday in 1987.

On October 19, 1987, Hong Kong’s Hang Seng Index plunged 46.2 percent. The collapse spread across international markets, with severe falls in Australia, the United Kingdom, the United States, and other financial centers. The Dow Jones Industrial Average lost 22.6 percent, still its largest one-day percentage decline.

The crash had several contributing explanations rather than one universally accepted cause. Rapid market growth, computerized trading, portfolio insurance, rising interest rates, and international currency tensions all featured in contemporary analyses. Market links also allowed fear to move rapidly between time zones.

The Hong Kong figure is often overlooked because “Black Monday” is most strongly associated with Wall Street. Hong Kong’s exchange closed for several days after the crash, and regulators later introduced reforms intended to strengthen market stability and trading procedures.