The Dow Jones has closed down more than 700 points as investors feared that trade tensions will spike between the US and China.
The Trump administration announced trade sanctions against China today, and Beijing has said it will defend itself.
Industrial and technology companies, which depend heavily on foreign trade, took some of the worst losses.
Boeing, Caterpillar and Microsoft all fell sharply while bond prices surged as investors sought cover, sending yields lower.
High-dividend stocks like utilities, another safe-play investment, rose.
US indexes suffered their worst drops since February 8. The S&P 500 index dropped 68 points, or 2.5%, to 2,643, erasing its gain for the year.
The Dow sank 724 points, or 2.9%, to 23,957 while the Nasdaq lost 178 points, or 2.4%, to 7,166.