WASHINGTON — Roger J. Stone Jr., an off-and-on adviser to President Trump for decades, has acknowledged that he had contact on Twitter with Guccifer 2.0, the mysterious online figure that is believed to be a front for Russian intelligence officials.

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It is the first time that someone associated with Mr. Trump has confirmed any type of contact with Guccifer 2.0, which claimed to be a Romanian hacker and took credit for the hacking of the Democratic National Committee.

But Mr. Stone insisted in an interview that the contact had been brief and involved nothing more than the exchange of a few direct messages, well after the party committee had been hacked. “Even if he is a Russian agent, my cursory exchange with him happens after he releases the D.N.C. stuff,” Mr. Stone said on Saturday. “There’s only one exchange with him. I had no further exchanges.”

Mr. Stone said the exchange took place after he had published an article on Aug. 5 on the Breitbart News site about the hacking, which the American government has tied to a Russian effort to meddle in the election. After Guccifer 2.0 had been suspended by Twitter, Mr. Stone posted a message against “censorship,” and he later had what he called an “innocuous” exchange over direct message.

“You would need a time machine in order to collude, and, as I said yesterday, Putin has not yet perfected a time machine,” he said, referring to the Russian president, Vladimir V. Putin.

Mr. Stone had exchanges about the direct messages with the website The Smoking Gun in mid-February; at the time, he said he did not recall the messages. On Friday, two days after the website published an article on the messages, Mr. Stone released screen grabs from his Twitter account to publications, including The Washington Times. It was unclear how The Smoking Gun had become aware of the messages.

American officials have said that Mr. Stone is one of several Trump advisers under scrutiny over their ties to Russia.

In August, Mr. Stone wrote on Twitter that John D. Podesta, Hillary Clinton’s campaign chairman, would soon go through his “time in the barrel.” Weeks later, WikiLeaks began publishing a trove of Mr. Podesta’s hacked emails, the daily release of which was seen as damaging to the campaign.

While Mr. Stone said last summer that he had communicated with Julian Assange, the WikiLeaks founder, he later clarified that he had been in contact through an intermediary. WikiLeaks has said it did not receive the email trove from the Russians.

Mr. Stone, a subject of fascination in Republican politics for decades, parted company with the Trump campaign in the summer of 2015. He still advises Mr. Trump at times, and shares the president’s view that Democrats’ allegations of collusion between the campaign and Russia are a smear.

He denied any knowledge of what the hackers were up to before their attacks. “This is a witch hunt,” Mr. Stone said. “It’s the worst form of McCarthyism. Seems as if you’re not for nuclear war with the Russians over Syria, then you must be a traitor.”

American intelligence agencies believe that Guccifer 2.0 was an online persona used by Russia’s military intelligence service, the GRU, to spread emails and other information stolen from the Democratic National Committee.

In an assessment of Russian election meddling released in January, American intelligence agencies said they had “high confidence” that Guccifer 2.0 was a persona through which Russian intelligence officials sought “to release U.S. victim data obtained in cyberoperations publicly and in exclusives to media outlets and related material to WikiLeaks.”

The report, though strongly worded, offers little direct evidence, allowing questions to fester about the precise origins of Guccifer 2.0, and what was known by Mr. Stone and others who dealt with the hackers.

During the campaign, Guccifer 2.0 used social media to invite individual reporters and Republican operatives to request specific caches of documents.

Not long after, reports began circulating that Russia was behind the hacks, and that the materials were being spread as part of a campaign to undercut the candidacy of Mrs. Clinton. Still, that did not stop journalists and Republican operatives from dealing with Guccifer 2.0, and Mr. Stone was hardly alone in having contact with the hackers.