Stocks sold off to start the week Monday after President Donald Trump posted letters to countries indicating new tariffs on imported goods.

The Dow Jones Industrial Average tumbled 422.17 points, or 0.94%, and ended at 44,406.36. The S&P 500 fell 0.79% to close at 6,229.98, and the Nasdaq Composite lost 0.92%, settling at 20,412.52. It was the worst day since mid-June for all three of the major averages.

Imports from at least seven countries will face steep tariffs beginning Aug. 1, Trump revealed in a series of Truth Social posts on Monday. Trump shared screenshots of signed form letters to the leaders of South Korea, Japan, Malaysia, Kazakhstan, South Africa, Laos and Myanmar, that dictated new tariff rates for each nation.

The major averages hit their lows of the day in afternoon trading.

White House press secretary Karoline Leavitt said that 14 letters will be sent on Monday, with more to be expected in the following days. She added that Trump will sign an executive order to delay his tariff deadline, previously anticipated to be sometime this week, until Aug. 1.

Shares of Toyota Motor dropped 4% and Honda Motor fell 3.9% following Trump's posts. Nvidia was slightly lower, while Apple and Alphabet lost more than 1%. AMD shed over 2%.

These mark the first of several trade announcements Trump was expected to make. Treasury Secretary Scott Bessent said Monday on CNBC's "Squawk Box" that there would be several trade announcements over the next 48 hours, adding that he expects "it's going to be a busy couple of days."

Adding to trade worries, Trump also threatened an additional 10% tariff on countries that align with the "Anti-American policies of BRICS," which refers to emerging market countries including Brazil, Russia, India and China. Trump did not elaborate on any specific policy of BRICS.

The announcement came as the group met in Rio de Janeiro, Brazil. In recent years, the group has sought to move away from its dependence on the U.S. dollar.

"Given that we've got this optimistic view — the markets are at record highs — tariff talk is not going to be helpful," Jed Ellerbroek, portfolio manager at Argent Capital Management, told CNBC in an interview. "The more we're talking about tariffs, the less happy the market is."

Tesla shares also put pressure on the broader market. The EV maker shed nearly 7% after CEO Elon Musk announced his intention over the weekend to form a new political party called the "America Party." Investors have been less than pleased with the billionaire's foray into politics this year, which some say have damaged Tesla's brands and sales.

— CNBC's Erin Doherty and Kevin Breuninger contributed to this report.