The move marked the latest leg of a monthslong slide that has dragged down a swath of other assets, from relatively safe corporate bonds to speculative investments such as cryptocurrencies and shares of unprofitable tech companies.

Rising Treasury yields, which are largely influenced by expectations for future Fed policies, push up borrowing costs across the economy. They also can hurt demand for riskier stocks by reducing the value that investors place on their future earnings.

On Wednesday, investors got a rare reprieve when Mr. Powell helped lift both stock and bond prices by saying officials weren’t giving serious consideration to a rate increase of three-quarters of a percentage point, after the central bank had just lifted rates a half a percentage point.

On Thursday, though, some investors and analysts emphasized that despite Mr. Powell’s comments, inflation is still running extremely hot and the Fed is still expected to raise its benchmark federal-funds rate by half a percentage point at each of its next two policy meetings. A few said investors are now even more worried that the central bank wouldn’t do enough in the near term to control inflation and would therefore have to raise rates even higher over the longer term.

Investors “are calling the Fed’s bluff,” said Priya Misra, head of global rates strategy at TD Securities in New York. Essentially, she said, they are “saying inflation is a problem so [the] Fed will need to hike more and kill the economy,” though she added she wasn’t sure why the selling was so concentrated on Thursday in particular.

It already has been a very hard 2022 for bond investors. The Bloomberg U.S. Aggregate bond index—largely U.S. Treasurys, highly rated corporate bonds and mortgage-backed securities—has returned minus 9.4% this year as of Wednesday.

On Thursday, the yield on the benchmark 10-year U.S. Treasury note settled at 3.066%, its highest close since November 2018, up from 2.914% Wednesday. The yield on the 30-year bond climbed even further, logging its biggest one-day increase in more than two years to close at 3.159%.

As the bond rout deepened, Mohit Bajaj, director of ETF trading solutions at WallachBeth Capital, said he was seeing money rush out of long-term corporate bonds and junk bonds and into less-risky forms of debt. The

SPDR Portfolio Long Term Corporate Bond ETF

slid 2.7%, its worst one-day decline since the pandemic-fueled market selloff on March 19, 2020. The iShares High Yield Bond Factor ETF shed 1.9%, its worst one-day decline since June 11, 2020.

Some investors cautioned against reading too much into Thursday’s yield jump. Early in the day, several analysts said the increase in Treasury yields could at least partially be ascribed to a large batch of new corporate bond sales that were announced in the morning. Large, sudden moves in bond prices also are often exacerbated by a variety of technical factors, including hedge funds being forced to close out bets as yields cross certain thresholds.

In the coming days, trading in Treasurys is likely to be more grounded in economic data, with the release of jobs data on Friday and the consumer-price index on Wednesday.

Yields on Treasurys largely reflect the average federal-funds rate that investors expect over the life of a given bond. As it stands, interest-rate derivatives indicate investors expect the fed-funds rate to reach 3.5% next year from its current range between 0.75% and 1%.

That forecast, though, masks a range of views—with many investors still hopeful that inflation could slow down enough to keep short-term rates under 3% while others think they may have to top 5%.

If the past two days proved anything, it is that investors, at the moment, are filled with uncertainty and lack the conviction to hold “one position for any length of time,” said Christopher Sullivan, chief investment officer at United Nations Credit Union.

Such swings in sentiment, he added, should be “a feature of the market for a while.”

—Akane Otani contributed to this article.