Long-term U. S. government bond yields topped 3% for the first time in more than four years, a sign investors’ confidence in the stability of economic growth is outpacing fears about the longevity of the postcrisis expansion.

It is a climb with significant implications for financial markets. The 10-year yield is a barometer that influences borrowing costs for consumers, corporations and state and local governments. Its half-percentage point climb to similar heights earlier this year contributed to the tumble in the Dow Jones Industrial Average in February, as higher yields dented investors’ confidence that stock valuations could rise unceasingly.

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Stocks have stabilized since, buoyed recently by strong corporate earnings. That relative calm, along with the climb in yields, suggests investors are moving past their longstanding worry that economies world-wide would prove unable to break out of a rut of weak wage growth and inflation and keep interest rates depressed for years to come.

The rise in the yield is happening as the Federal Reserve has reached what some investors say is roughly the halfway point in its cycle of rate increases. Policy makers at their March meeting signaled an intent to raise rates two more times this year and three times in 2019. The yield on the benchmark 10-year U. S. Treasury note recently traded at 3.001%, according to Tradeweb, its highest intraday level since January 2014. That is up from 2.973% the previous day and 2.41% at the end of 2017. Yields rise as bond prices fall.

Investors had felt they were “fated to be in this low interest-rate environment forever and ever, ” said Wan Chong-Kung, who manages bond portfolios with Nuveen Asset Management. “That’s part of what lends the 3% level the mystique it has. ”

The 10-year yield has topped 3% several times since the financial crisis, only to soon retreat below the level. That pattern has left investors debating whether this latest surge marks a new phase in the recovery or the latest in a series of false starts. Yields stayed near historic lows in the postcrisis years as the Fed undertook quantitative easing on a massive scale and policy makers saw greater risk in a sluggish recovery and the threat of deflation than in the potential for monetary stimulus to spur rapid inflation.

A surge above 3% could spook some investors—particularly those who for years have said low yields justified high stock valuations—while raising corporate borrowing costs and even mortgage rates. But with consumers less exposed to debt than during the last expansion, the threshold’s crossing by itself is unlikely to prove economically significant, said Michael Cloherty, head of interest-rate strategy at RBC Capital Markets.

The rise in the yield is happening as the Federal Reserve has reached what some investors say is roughly the halfway point in its cycle of rate increases. Photo: Andrew Harnik/Associated Press

Signs of inflationary pressures helped spur some of the recent selling in 10-year Treasurys, several analysts said. U. S. oil prices are approaching $70 a barrel for the first time since 2014, and trade tensions with China also threaten to increase prices. Inflation poses a threat to the value of government bonds because it chips away at the purchasing power of their fixed payments and can push the Fed to raise interest rates.

The flattening in the yield curve—the distance between short- and longer-term yields—is one signal investors have greater confidence in the Fed’s pace of rate increases than the longer-term outlook for the economy. Two-year yields tend to rise along with investors’ expectations for tighter Fed policy, while longer-term yields are more responsive to the outlook for growth and inflation.

The prospect of Fed rate increases has been driving two-year yields higher, even as the 10-year yield has traded within a narrow range since it last approached 3% in February. That has narrowed the spread between the two yields to about 0.5 percentage point as of Monday, down from 2.65 percentage points at the end of 2013.

Fed-funds futures, used by investors to place bets on central-bank policy, late Monday suggested a 48% probability the Fed would raise rates three more times this year, up from 33% a month ago, according to CME Group data.

“We’re seeing the market saying there’s more” to a potential slowdown in growth than just “trade risks, ” said Adrian Helfert, deputy head of global aggregate investing at Amundi Pioneer. The pace of flattening suggests the yield curve could invert in early 2019, with a recession following a little more than a year after, he said.

Because short-term rates have exceeded longer-term rates before each recession since at least 1975—a phenomenon known as an inverted yield curve—investors become wary as the curve flattens. Yet the flattening has occurred while economic growth continues to be steady, and few analysts see signs of any imminent slowdown.

The Fed is likely to raise interest rates four times in 2018, in part because strong economic fundamentals, but also to give the bank leeway to cut interest rates when the economy enters a slowdown, said Doug Peebles, chief investment officer at Alliance Bernstein Fixed Income. With the added economic stimulus from the 2017 tax cut, “it’s just going to solidify the path the Fed has already started on, ” he said.

While investors debate the meaning of the flattening curve, one factor that could propel yields higher is increased government borrowing. This year’s rise in yields came after the passage of $1.5 trillion in tax cuts and the bigger bond sales the government needs to finance them. The Congressional Budget Office forecasts that the deficit will top $1 trillion in 2020 and remain above that level for the foreseeable future.

At the same time, analysts see factors that could cap the climb. Because the 10-year yield is a benchmark rate used to help set interest rates for different kinds of loans, including mortgages, auto loans and corporate debt, higher yields increase the cost of debt, which can act as a brake on the economy. Indeed, yields remain low by historical standards and the implied inflation forecast from inflation-indexed Treasurys recently hit a 2018 high of 2.19% for the next 10 years.

While the financial crisis happen almost 10 years ago, the long path to 3% shows that the pain is only “slowly wearing off, ” said Jeremy Siegel, professor of finance at the University of Pennsylvania’s Wharton School.

Write to Daniel Kruger at Daniel. Kruger@wsj. com