Taylor Dudley had been held since April 2022 and was one of several detained Americans whom Richardson and US authorities have been seeking to have freed.

According to CNN, Dudley, 35, had crossed into the Kaliningrad, an exclave between Poland and Lithuania, from Poland where he had been attending a music festival.

"It is significant that despite the current environment between our two countries, the Russian authorities did the right thing by releasing Taylor today," Richardson said in a statement.

The release came one month after Washington swapped jailed Russian arms dealer Viktor Bout for US basketball star Brittney Griner, whom the US said had been wrongfully detained and imprisoned for possessing vape cartridges with cannabis oil.

In Dudley's case, the US and Russian governments had not publicized his detention and Washington did not say he was wrongfully detained.

Richardson made no mention of any swap involved in his release.

He said a team from the Richardson Center, which has helped free detained Americans from a number of countries, had visited Moscow several times in the past year on Dudley's case.

"The negotiations and work to secure Taylor's safe return were done discreetly and with engagement on the ground in both Moscow and Kaliningrad and with full support from Taylor's family back in the United States," the statement said.

"As we celebrate Taylor's safe return, we remain very concerned for Paul Whelan and committed to continue to work on his safe return," Richardson said.

Whelan is a former US Marine who was arrested in Moscow in 2018 and then convicted of spying.

The US says he was a private citizen visiting Moscow on personal business, and has demanded his release.

He was sentenced in 2020 to 16 years in a Russian prison.

US efforts to negotiate his freedom as part of the Griner deal failed after Moscow demanded in exchange the release of a Russian former intelligence official imprisoned in Germany for assassinating an anti-Moscow Chechen man in Berlin in 2019.