Amazon.com Inc. projected sales in the current quarter would be far below expectations, sending its stock plunging and offering the latest stark sign of how shifting economic forces are battering tech giants that thrived during the pandemic.

The company on Thursday said sales in the recently completed third quarter rose 15% from a year earlier, while net income was $2.9 billion—its first quarterly profit in 2022, though still a 9% decline from the same period last year.

The e-commerce giant jolted investors with its projection for revenue of $140 billion to $148 billion in the current period—analysts had expected more than $155 billion, according to FactSet. Amazon, which said the estimate includes a sizable hit from foreign-exchange factors, also said it anticipated operating income of anywhere between zero and $4 billion, reflecting the uncertainty looming over what is traditionally its biggest quarter of the year because of holiday shopping.

The company’s shares fell more than 12% in after-hours trading following the results to trade near $97. At that level, Amazon’s valuation is below $1 trillion, which it first hit in 2018.

The disappointing outlook capped an extraordinary several days that also saw shares of other tech giants plummet after their results showed worsening conditions in a range of areas.

Shares of Facebook parent Meta Platforms Inc., already battered over the past year, dropped nearly 25% on Thursday after it reported its second quarterly revenue decline in a row a day earlier. Microsoft Corp.’s stock also fell after it delivered on Tuesday its worst net income decline in more than two years and the weakest revenue growth in over five years. Google-parent Alphabet Inc. similarly disappointed investors with slowing sales.

These tech companies flourished during the pandemic, as life and work suddenly shifted more to the internet, pushing up sales and spurring the already fast-growing companies to accelerate hiring and investment.

Now, one after another, engines that drove that growth are sputtering. Sales of personal computers and other gadgets are falling. Consumers, walloped by inflation, are broadly trimming their spending, while companies are tightening their outlays for everything from digital ads to IT services.

“There is obviously a lot happening in the macroeconomic environment, and we’ll balance our investments to be more streamlined without compromising our key long-term, strategic bets,” Amazon Chief Executive Andy Jassy

said Thursday. 

In the third quarter, Amazon’s online store sales rose 7% to $53.48 billion after falling in recent quarters. The segment includes product sales primarily on its flagship site and digital media content. Its online sales got a boost from its annual Prime Day sale, which this year fell in the third quarter where last year it was in the second quarter.

While still the nation’s largest online store, Amazon’s e-commerce division has struggled to grow this year. The company in the second quarter reported a 4% year-over-year drop in its online stores segment. That marked the largest drop since the metric was first reported in 2016.

This year, Amazon’s e-commerce machine—which has grown at breakneck speed for decade—has been showing signs that it could be entering a phase of slower growth. After a multibillion-dollar infrastructure build-out and hiring spree, it now has to contend with high inflation and concerns about a recession weighing on consumer spending.

Chief Financial Officer

Brian Olsavsky said the company has entered a period of caution.

“We are preparing for what could be a slower growth period like most companies. We are going to be very careful on our hiring,” Mr. Olsavsky said during a call with reporters Thursday. “We certainly are looking at our cost structure and looking for areas where we can save money.”

He said Amazon is “seeing signs all around that people’s budgets are tight, inflation is still high.”

Analysts say the new challenges Amazon faces in e-commerce could linger.

Amazon has the largest share of online commerce, about 38%, but its market share has plateaued in recent years, according to market research firm Insider Intelligence. Analysts say the company’s size has made it unlikely the e-commerce unit’s growth would hit the same pace it once did. Amazon also is dealing with increased competition from

Walmart Inc., Target Corp. and others.

Mr. Jassy has shifted toward cost-cutting. The company cut back on subleasing millions of square feet of excess warehouse space and put off opening new facilities while earlier thinning out its hourly workforce through attrition.

It enacted a hiring freeze through the end of the year at its corporate retail division, the segment that drives core sales and is responsible for a large part of this year’s slowdown. The company has paused hiring among some teams at its Amazon Web Services cloud-computing division.

While Amazon’s earnings continue to be aided by AWS and its expanding advertising business, growth slowed in the cloud business. AWS had sales of $20.5 billion during the third quarter, a 27% rise but one of the lowest rates of growth posted by the unit in recent quarters. Mr. Olsavsky said the company saw AWS customers “working to cut their bills.”

Amazon’s advertising revenues rose 25% to $9.5 billion.

Amazon is headed toward the end of the year with added challenges. After needing fewer blue-collar employees earlier in the year, it has looked to add more than 100,000 workers at its warehouses to meet the expected holiday demand. Still, that strategy has come with a cost. Amazon recently said it would spend $1 billion to raise average starting salaries to $19 an hour nationwide and is earmarking millions to raise wages and benefits for its delivery employees.

Consumers will be more likely to return to bricks-and-mortar stores for their holiday shopping this year, and economic concerns will likely weigh on spending, according to analysts. Amazon’s own Jeff Bezos seemed cautious about the future. He recently said it is time to “batten down the hatches,” referring to warning signs that the U.S. is headed for a recession.

Write to Sebastian Herrera at sebastian.herrera@wsj.com