Putin confidant Viktor Medvedchuk is the most prominent captive released in the prisoner swap.

Viktor Medvedchuk, the most prominent captive released by Ukraine in a prisoner swap with Russia, is a close friend of President Vladimir V. Putin of Russia who had acted as the Kremlin’s primary agent of influence in Ukraine for years.

Mr. Medvedchuk, a Ukrainian politician and oligarch, was handed over alongside Russian pilots and senior military officials, a top adviser to President Volodymyr Zelensky said on Wednesday, in exchange for more than 200 Ukrainian fighters including commanders of the Azov Battalion, who have been celebrated as heroes in Ukraine for their last-stand defense of Mariupol.It was the largest prisoner swap in the seven-month long war.

Russian officials had previously disavowed any claims to Mr. Medvedchuk, despite his long-known ties to Mr. Putin, who is the godfather of Mr. Medvedchuk’s daughter. A Kremlin spokesman in May had dismissed the idea of exchanging Mr. Medvedchuk with Ukrainian fighters, saying that he “has nothing to do with Russia,” according to Russian state media.

Mr. Medvedchuk was captured by authorities in Ukraine in April after he fled house arrest while awaiting trial on treason charges in a case initiated last year. After his detention, officials in Ukraine also seized some of his vast wealth, amassed through energy deals with the Kremlin while working in support of Russian interests in Ukraine and beyond.

At the time of the arrest, Mr. Zelensky posted a photo showing Mr. Medvedchuk in handcuffs, looking disheveled. “Let Medvedchuk be an example for you,” the Ukrainian leader said in a nightly address. Even the former oligarch did not escape.”

A former deputy speaker of Ukraine’s Parliament, a presidential adviser and a negotiator in prisoner exchanges with Russia, Mr. Medvedchuk was a polarizing figure in Ukraine who championed a closer relationship with Moscow. His two-decade friendship with Mr. Putin is well documented. The Russian president visited Mr. Medvedchuk’s lavish Crimea residence in 2012, and an official Kremlin photograph showed Mr. Medvedchuk with Mr. Putin at a martial arts tournament in 2013.

Mr. Medvedchuk’s influence was such that his name emerged in the federal investigation into Russian meddling in the 2016 U.S. presidential election. He was a client of Republican political consultant Paul J. Manafort, who advised pro-Russian Ukrainian politicians before becoming chairman of Donald J. Trump’s election campaign.

At the time, Ukrainian authorities released entries from an accounting document showing that Mr. Manafort had received $12.7 million from pro-Russian politicians in Ukraine. In the middle of the investigation into Russian meddling in 2017, Reuters reported that the F.B.I. was examining phone calls and text messages between people close to Mr. Trump and people with ties to Mr. Putin, including Mr. Medvedchuk.

Mr. Medvedchuk has denied wrongdoing, saying Mr. Manafort only advised his political party on electoral strategy.

His transfer to Russia is likely to mean he will not stand trial on charges that he faces in Ukraine, or be interviewed by investigators from other countries looking into Russian influence peddling.

Mr. Zelensky’s adviser, Andriy Yermak, said in a statement that it was a worthwhile trade and that Mr. Medvedchuk “had already given all the testimony he could.”