U.S. and Japanese officials confirmed that the Treasury Department had moved last week to prop up the currency, which had been trading near four-decade lows.

The United States and Japan last week carried out a rare coordinated intervention in foreign exchange markets to stabilize the rapidly weakening yen, marking the first joint operation of its kind in more than a decade.

President Trump confirmed on Sunday that the Treasury Department had assisted Japanese authorities. In a statement, Satsuki Katayama, Japan’s finance minister, said the intervention was taken to counter “excessive volatility and disorderly movements” in the yen after months of sustained weakness.

The yen has depreciated steadily against the U.S. dollar this year, as investors have grown increasingly concerned about Japan’s expanding fiscal spending, amplified by higher energy costs stemming from the U.S. war with Iran. In July, the currency fell to its weakest level since the early 1980s, with the dollar briefly climbing above 163 yen, up from around 147 yen a year earlier.

The Financial Times, citing sources, reported last week that the United States had sold euros to buy yen, helping to strengthen the Japanese currency. In Tokyo trading on Monday, the yen surged to around 155 per dollar before settling at around 156.50.

The intervention underscores the delicate balance surrounding the U.S. dollar’s global role. President Trump has repeatedly argued that an overly strong dollar hurts American exporters by making U.S. goods more expensive abroad. At the same time, Washington has a strong interest in preserving the dollar’s position as the world’s dominant reserve currency — what businesses, banks and people use to price goods and settle accounts.

For Japan, a weak yen has produced mixed results. While it improves the competitiveness of Japanese exporters, it also increases the cost of imports, particularly energy. Because oil is typically priced in dollars, Japan has faced the double burden of higher global oil prices and a weaker currency, forcing importers to spend more yen for every barrel.

The yen’s decline mirrors broader weakness across several Asian currencies as the dollar has strengthened. Since April, Japan’s finance ministry has spent tens of billions of dollars purchasing yen to prop up the currency, with limited success.

Speaking in Tokyo on Monday, Ms. Katayama said the government believes the yen is undervalued, pointing to Prime Minister Sanae Takaichi’s efforts to revive the economy.

The coordinated intervention coincided with the Bank of Japan’s latest policy meeting, where officials left interest rates unchanged but signaled that additional rate increases remain possible. Japan’s central bank raised rates in June, with market analysts expecting another increase before year-end.

After decades of ultralow interest rates aimed at combating deflation, the Bank of Japan has been gradually raising rates in response to inflationary pressures. Ms. Takaichi, however, has championed low borrowing costs as part of an agenda built around fiscal stimulus, tax cuts and higher defense spending — priorities that become more difficult to finance as interest rates rise.

Many economists argue that higher Japanese interest rates, by narrowing the gap with U.S. rates, would help relieve downward pressure on the yen. Treasury Secretary Scott Bessent expressed a similar view during a visit to Tokyo earlier this year.

So far this year, Japanese stocks are performing better than their U.S. counterparts. Despite a decline over the past month, Japan’s benchmark Nikkei 225 index remains almost 30 percent higher this year, while the S&P 500 is up 9.2 percent.

The United States last coordinated currency intervention with Japan in 2011, when the two countries sought to weaken an unusually strong yen after it surged in the aftermath of Japan’s devastating earthquake and tsunami. Washington also joined Japan in supporting the yen during a period of sharp depreciation in 1998.

For the Trump administration, a persistently weak yen creates several challenges. It gives Japanese manufacturers a competitive advantage over U.S. producers, undercutting President Trump’s broader effort to use tariffs to encourage companies to shift production to the United States.

A weaker yen also risks fueling inflation in Japan, potentially prompting the Bank of Japan to raise interest rates more aggressively. Higher Japanese bond yields could encourage domestic investors to shift money out of U.S. Treasuries and back into Japanese assets, pushing Treasury prices lower, raising U.S. yields and increasing borrowing costs across the American economy.

Mr. Trump was asked on Sunday why the Treasury Department had taken the step this time.

“To support the Japanese currency,” he said, after he landed at Andrews Air Force Base in Maryland. “Because we have a good relationship with Japan. We’re very strong, very, very strong financially.”

He added: “Japan’s been very good to us, with the exception, of course, of Pearl Harbor.”

Mr. Bessent also acknowledged the intervention in a post on X that the coordinated foreign exchange actions had “countered disorderly yen movements,” he wrote, adding, “We will not hesitate to participate in further joint intervention.”

Ms. Katayama said Japan’s finance ministry “remains attentive and in close communication” with the Treasury Department.

Still, analysts cautioned that intervention alone is unlikely to reverse the yen’s decline unless underlying economic forces also change.

“The intervention highlights growing official determination to defend the currency, but equally underscores the limits of relying on FX operations alone,” analysts at Wells Fargo said.