Russia releases a U.S. Navy veteran quietly detained in Kaliningrad in April of last year.

Russia released a Navy veteran who had been detained since April in Kaliningrad, a Russian enclave between Poland and Lithuania, a spokesman for his family announced on Thursday, marking the second time in just over a month that an American has been freed from Russian custody.

Russian officials allowed Taylor Dudley, a 35-year-old U.S. citizen to cross the Polish border. Mr. Dudley had been held for nine months, the spokesman said, though his case was largely unknown to people outside the U.S. government, his family and its advocates.

Mr. Dudley was greeted in Poland by an official from the U.S. Embassy in Warsaw and Bill Richardson, a former New Mexico congressman and governor who specializes in negotiating the freedom of Americans detained overseas.

Mr. Dudley was backpacking in Europe and had traveled to Poland to attend a music festival, according to the spokesman, Jonathan Franks. He said Mr. Dudley “at some point crossed the Russian border” into Kaliningrad. It remained unclear on what charges he had been held.

It was also not clear what, if anything, the U.S. might have offered Russia in return for Mr. Dudley’s freedom, which came less than five weeks after Russia’s release of the WNBA star Brittney Griner in a prisoner swap organized by the Biden administration. A spokesman for Mr. Richardson told CNN, which first reported the news, that no exchange had been made by the United States.

The White House and State Department had no immediate official comment.

“It is significant that despite the current environment between our two countries, the Russian authorities did the right thing by releasing Taylor today,” Mr. Richardson said in a statement. He said he had been working “discreetly” with Mr. Dudley’s family for six months on the matter, along with U.S. State Department officials and abusinessman close to President Vladimir V. Putin of Russia, among others.

One White House official, speaking on background because of the sensitivity of the matter, credited the work of U.S. officials at embassies in Warsaw and Moscow and said that, out of consideration for the family’s privacy, the administration would not provide further details.

U.S. officials had not publicized Mr. Dudley’s case or declared him “wrongfully detained,” as they have others held in Russia since its Feb. 24 invasion of Ukraine, including Ms. Griner and the former Marine and corporate security consultant Paul Whelan.

Ms. Griner was released on Dec. 8 in exchange for the Russian arms dealer Viktor Bout, who had been serving a U.S. federal prison sentence. Mr. Whelan remains imprisoned on espionage charges that he and the U.S. government deny.

Biden officials say they have been working with Russia to secure his freedom but have reported no specific progress since Ms. Griner’s release. Many analysts believe that Russia would only trade Mr. Whelan for a captured Russian spy.

Mr. Richardson said in his statement on Thursday that “as we celebrate Taylor’s safe return, we remain very concerned for Paul Whelan and committed to continue to work on his safe return, as we have been for the last four years, as well as other Americans.”

Mr. Franks, in his statement, said, “Over the past year, our team traveled to Moscow and the region multiple times, liaising with our Russian counterparts and conduits.”

Both he and Mr. Richardson also credited a foundation run by the American entrepreneur Steve Menzies and Vitaly Pruss, a New York-based businessman with ties to Russia.