Tesla shares closed down 21.06%, making it the worst one-day loss on record. Tuesday's drop brought the company's market valuation down by roughly $82 billion to $307.7 billion. The stock has been on a tear this year, having risen around 300%, and the company is now worth more than some of the world's largest automakers, including Toyota and Volkswagen.

On Friday, the S&P 500 Index Committee decided to add e-commerce site Etsy, automatic test equipment maker Teradyne and pharmaceutical firm Catalent to the S&P 500, but stopped short of including Tesla. Some investors had expected Tesla to be included this quarter, after it reported its fourth consecutive quarter of profitability in July.

Tesla stock dropped more than 7% after hours on Friday following the news. U.S. markets were closed Monday because of Labor Day.

Tesla's move lower Tuesday also follows a major reversal in the big technology stocks last week, amid fears that valuations had reached unsustainable levels. Japanese tech investment juggernaut SoftBank was reportedly the mystery "Nasdaq whale" that bought billions of dollars in call options in Big Tech names, including Tesla, Amazon, Microsoft and Netflix, potentially driving up valuations. SoftBank declined to comment on the reports.