On Wednesday, the iPhone maker said it expected revenue of about $84bn (£67bn) for the last three months of 2018, down from a forecast of at least $89bn.
The news depressed other US shares. An hour into trading the Dow Jones and the Nasdaq index were down more than 2%.
Shares in fashion firm Burberry are down 5%. LVMH and Hermes were 2% lower.
Those companies are increasingly reliant on Chinese sales.
Like at other consumer goods companies, the festive season is typically Apple’s strongest quarter, but revenues of $84bn would mark an almost 5% fall from the same period last year and represent the firm’s first year-on-year quarterly decline since 2016.
Wednesday’s cut to the sales forecast marked the first time Apple has revised its guidance to investors in more than 15 years, prompting the share price plunge.
Apple, which was until recently the largest publically trade company in the US, is now worth less than Microsoft, Amazon and Google’s parent company, Alphabet.
Troubles in China
In a letter to investors on Wednesday, chief executive Tim Cook said the firm’s sales problems were primarily in its Greater China region, which includes Hong Kong and Taiwan and accounts for almost 20% of its revenue.
“While we anticipated some challenges in key emerging markets, we did not foresee the magnitude of the economic deceleration, particularly in Greater China,” he said.
However, he added that developed markets saw troubles as well, as fewer customers than expected chose to upgrade to Apple’s newest phones.
It appeared to confirm doubts about the firm’s prospects that have troubled investors in recent months, contributing to the broader market sell-off.
Production cuts by major suppliers had led to worries that the firm’s newest phones were not gaining traction among buyers, in part due to high prices.
“The question for investors will be the extent to which Apple’s aggressive pricing has exacerbated this situation and what this means for the company’s longer-term pricing power within its iPhone franchise,” said James Cordwell, an analyst at Atlantic Equities.
The firm had warned investors in November that a strengthening dollar and economic weakness in some overseas markets would be likely to hurt sales in the last three months of the year.
On Wednesday, Apple said trade tensions had hurt consumer confidence.
“As the climate of mounting uncertainty weighed on financial markets, the effects appeared to reach consumers as well, with traffic to our retail stores and our channel partners in China declining as the quarter progressed,” Mr Cook wrote in the letter.
He added that Apple was taking steps to make it easier for customers to trade in their phones and said other parts of the firm’s business, including services, remained strong.
“While it’s disappointing to revise our guidance, our performance in many areas showed remarkable strength in spite of these challenges,” he said.