WASHINGTON — When the White House announced this week that President Trump had sent President Xi Jinping of China a letter wishing him a happy Chinese New Year, it did not disclose a major reason for the friendly gesture: Mr. Xi had not, at that point, gotten on the phone with Mr. Trump.

That changed on Thursday night, when the two leaders had what the White House described as a “lengthy conversation” during which Mr. Trump agreed to honor the “One China” policy the president had previously refused to commit to uphold.

Stung by an earlier, and unorthodox, telephone call between Mr. Trump and the president of Taiwan, Mr. Xi had not spoken to the American leader since Nov. 14, the week after he was elected. Administration officials had predicted a conversation would come only after Mr. Trump publicly committed to recognizing a single Chinese government in Beijing.

And so Mr. Trump, during what the White House called “a lengthy telephone conversation,” formally and officially bent to Beijing.

“The two leaders discussed numerous topics and President Trump agreed, at the request of President Xi, to honor our One China policy,” the White House said in a statement released late Thursday night. It said the discussion was “extremely cordial” and that the presidents “extended invitations to meet in their respective countries.”

A day earlier, on Wednesday, the White House took the unusual step of releasing Mr. Trump’s letter to wish Mr. Xi an happy new year. It marked the start of an effort to keep the United States-China relationship from unraveling further, several officials said, especially as Mr. Trump is about to welcome Japan’s prime minister for an extravagant three-day visit that will include a weekend of golf in Florida.

Relations between Washington and Beijing had been frozen since December, when Mr. Trump took a congratulatory phone call from Taiwan’s president, Tsai Ing-wen. The United States has not had diplomatic relations with Taiwan since 1979, but Mr. Trump defended the call by saying he did not know why the United States should be bound by the One China policy.

To lay the groundwork for a better relationship, Mr. Trump’s national security adviser, Michael T. Flynn, spoke last Friday to China’s top foreign policy official, Yang Jiechi. That call produced only a vague commitment to “reinforce high-level exchanges,” suggesting that Mr. Trump’s statements on China still precluded a direct leader-to-leader exchange.

As a gesture of conciliation, Mr. Flynn and his deputy, K. T. McFarland, hand-delivered the letter to China’s ambassador to the United States, Cui Tiankai. Mr. Trump wrote that he wished “the Chinese people a happy Lantern Festival and prosperous Year of the Rooster.” He also said he “looks forward to working with President Xi to develop a constructive relationship that benefits both the United States and China.”

“This letter means they’re looking for creative ways to stabilize this relationship when Trump and Xi can’t talk due to differences over Taiwan policy,” said Evan S. Medeiros, who was senior director for Asia in the National Security Council during the Obama administration.

On Thursday, Secretary of State Rex W. Tillerson was at the White House for meetings to discuss whether the White House should publicly reaffirm its commitment to the One China policy as a way of breaking the deadlock and getting the two presidents back on the phone.

Mr. Tillerson’s involvement in the issue is noteworthy because he pledged, in written answers to questions after his Senate confirmation hearing, to uphold the One China policy. He specifically rejected the idea, advanced by Mr. Trump, that Taiwan be used as a bargaining chip in a broader negotiation with China on trade and other issues.

As recently as December, in an interview with Fox News, Mr. Trump said the policy should be contingent on extracting other concessions from Beijing.

 “We’re being hurt very badly by China with devaluation; with taxing us heavy at the borders when we don’t tax them; with building a massive fortress in the middle of the South China Sea, which they shouldn’t be doing; and, frankly, with not helping us at all with North Korea,” he said.

Since his inauguration, Mr. Trump has spoken by phone. with about 20 foreign leaders. Although these calls are typically highly scripted affairs, Mr. Trmp’s have been anything but. His conversation last month with Prime Minister Malcolm Turnbull of Australia turned ontentious when Mr. Turnbull urged Mr. Trump to honor an agreement made under President Barack Obama to accept 1,250 refugees from an offshore detention center. 

But arguably no bilateral relationship is more important than the one between Beijing and Washington, and the fact that Mr. Trump and Mr. Xi until Thursday had not talked since Mr. Trump took office in January has drawn increasing scrutiny.

“The U.S.-China relationship only works if the two leaders have a serious relationship and use their contact to do real business,” Mr. Medeiros said. “Given the rigidity of the Chinese system, leader-level contact provides essential stability, direction and momentum to U.S.-China ties.”

Administration officials are also keenly aware that the Chinese will be closely watching the visit of Prime Minister Shinzo Abe of Japan, which begins here Friday with an Oval Office meeting, a White House lunch, and a joint news conference. Then Mr. Trump will take Mr. Abe to Palm Beach, Fla., on Air Force One, where the leaders plan to play golf at Mr. Trump’s club. They and their wives will also have dinner at Mr. Trump’s club, Mar-a-Lago.

To the status-conscious Chinese, this red-carpet treatment will not go unnoticed. Analysts say it may reinforce their suspicion that the Trump administration is making Japan the centerpiece of its Asia strategy.

In Beijing, Lu Kang, a spokesman for China’s Foreign Ministry, expressed thanks for Mr. Trump’s letter. He had dismissed as “senseless” speculation that Mr. Trump was snubbing Mr. Xi by not scheduling a phone call earlier. “The two countries share wide common interests, and cooperation is the only correct path for both,” Mr. Lu told reporters on Thursday.

Mr. Trump and his advisers have markedly shifted their tone toward China since he took office.

During the campaign, Mr. Trump advocated a 45 percent tariff on Chinese exports to the United States, complaining that China manipulated the value of its currency. Last week, however, the president’s daughter Ivanka attended a Lunar New Year celebration at the Chinese Embassy in Washington. Her daughter Arabella sang a New Year’s greeting in Mandarin that was widely viewed in China.

Ms. Trump’s husband, Jared Kushner, who is a senior adviser to Mr. Trump, met with Mr. Cui before the embassy event, part of a blossoming dialogue between the two men.

Mark Landler reported from Washington, and Michael Forsythe from Hong Kong. Maggie Haberman contributed reporting from Washington.