What was the fastest bear market in S&P 500 history, measured by the number of trading days to fall 20%?

The 2020 COVID-19 crash was the fastest S&P 500 bear market, reaching a 20% decline in 16 trading days.

The benchmark entered bear-market territory on March 12, 2020, after investors rapidly repriced shares as the COVID-19 pandemic spread and governments imposed restrictions. The speed of the move made it one of the most abrupt collapses in modern market history.

The decline followed a long bull market and affected energy, travel, finance, and many other sectors. Extraordinary fiscal and monetary support, along with changing expectations about vaccines and reopening, contributed to a powerful rebound from the March low.

A bear market is conventionally defined as a decline of at least 20% from a recent closing high. The 2020 classification therefore refers to the speed of the threshold crossing, not necessarily the largest percentage loss of all crashes or the longest-lasting downturn.