Hamas, the Palestinian armed group, said on Sunday night that it would free the last living American citizen held captive in Gaza, just days before President Trump is expected to arrive in the region for the first major foreign tour of his second term.

Khalil al-Hayya, Hamas’s lead negotiator, said in a statement that Hamas had agreed to free the hostage, Edan Alexander, 21, after talks with the United States. He did not say when Mr. Alexander would be released or what Hamas expected to receive in exchange.

Raised in Tenafly, N.J., Mr. Alexander, an Israeli American dual citizen, moved to Israel to serve in the military after high school. During the Hamas-led attacks on Oct. 7, 2023, Palestinian militants abducted him from the military post where he was stationed.

The announcement comes at a pivotal moment in the Middle East, where Mr. Trump is scheduled to land on Tuesday for a round of diplomacy. Mr. Trump is expected to visit Saudi Arabia, the United Arab Emirates and Qatar in the wake of disagreements with Israel over talks with Iran on its nuclear program.

The war in Gaza will hang heavy over Mr. Trump’s visit, even though he is not visiting Israel. The Israeli government is threatening a major military offensive that would displace most people in Gaza unless Hamas surrenders and turns over its remaining hostages. Israel has already been blocking food and other aid to Gaza for more than two months, deepening the enclave’s humanitarian crisis.

Hamas has broadly refused to free more captives, saying that Israel must first commit to a path to ending the war. But in a possible effort to gain favor with Mr. Trump, Hamas agreed to free Mr. Alexander as a gesture of good will, according to the U.S. official and another diplomat briefed on the talks, who spoke anonymously to discuss the sensitive negotiations.

The exact timing and mechanism of Mr. Alexander’s release were still unclear. The Trump administration hoped that he would be freed as soon as Monday, the U.S. official said. Mahmoud Mardawi, a Hamas official, said the release would take place in the next day or two.

During the talks, the United States pledged to undertake “great efforts” to end the war in Gaza, Mr. Mardawi said.

“We were asked to release Alexander and we complied with the request,” he said in a phone interview. The Americans, he said, had conveyed to Hamas that “the war ran its course” and “no longer had any justification.”

Mr. Alexander was one of about 250 people taken hostage during the Oct. 7 attack, which ignited the war in Gaza. The hostages were taken to Gaza for use as bargaining chips in future negotiations with Israel. More than 18 months later, 59 of them remain in the enclave. Dozens of them, including four U.S. citizens, are presumed by the Israeli authorities to be dead.

Mr. Alexander’s parents, Adi and Yael Alexander, have campaigned tirelessly for his release, meeting with officials and speaking at rallies. “We are living this day over and over,” Adi Alexander said in a February interview, referring to the Oct. 7 attacks.

Mr. and Ms. Alexander were traveling to Israel on Sunday night with Adam Boehler, Mr. Trump’s special envoy for hostage response, Mr. Boehler said in a phone call. Steve Witkoff, Mr. Trump’s Middle East envoy, and Secretary of State Marco Rubio were instrumental in securing Mr. Alexander’s release, he said.

Mr. Alexander’s family, in a statement, called his return “the greatest gift imaginable” and urged the Israeli government to negotiate the release of the remaining captives. “No hostage should be left behind,” they said.

Mr. Trump called news of Mr. Alexander’s impending release “a step taken in good faith towards the United States” and said, “Hopefully this is the first of those final steps necessary to end this brutal conflict.”

This year, the Trump administration broke with a longstanding American policy of boycotting Hamas, which the United States has designated as a terrorist group. Mr. Boehler held direct talks with Hamas leaders in Doha, Qatar, in an attempt to secure Mr. Alexander’s freedom, as well as the bodies of the four dead Americans. But they foundered amid Israeli objections.

In previous rounds of negotiations, Hamas set a clear price for freeing more hostages: Israel must agree to ultimately end the war, withdraw its forces from the Gaza Strip and release large numbers of Palestinian prisoners.

During the two-month cease-fire that began in January, Hamas handed over 30 hostages and the bodies of eight others, while Israel freed more than 1,500 Palestinians being held in its prisons. Israel ended the truce in mid-March, saying talks to secure the next steps in the agreement were deadlocked.

On Sunday night, the office of Prime Minister Benjamin Netanyahu said Washington had formally notified Israel that Mr. Alexander’s release would be a “gesture to the Americans” without any “compensations or conditions.” The U.S. official and the diplomat said Hamas had agreed to release Mr. Alexander without specific demands in exchange.

Mr. Netanyahu’s office suggested, however, that Mr. Alexander’s release would not lead Israel to pause its military campaign in Gaza, at least for the time being. “Under Israeli policy, negotiations will take place as fighting continues,” his office said.

The announcement that the United States had secured a promise of freedom for Mr. Alexander prompted hope in Israel. But it also led some Israelis to voice frustration with their own government, which has failed to gain the freedom of the remaining hostages.

Critics of Mr. Netanyahu have called on him to agree to an immediate agreement to end the war with Hamas and free the rest of the hostages. Mr. Netanyahu has held out, arguing that saving the captives is less important than “victory over our enemies.”

“The responsibility lies with the Israeli government,” said the Hostage Families’ Forum, an advocacy group which calls for an agreement with Hamas. “No one should be left behind.”

Isabel Kershner and Fatima AbdulKarim contributed reporting.