MOSCOW — Aleksei A. Navalny, the imprisoned Russian opposition leader, has been denied visits from his doctors and lawyers.

But one unlikely visitor to the notoriously harsh penal colony where he is being held did turn up this week: Maria Butina, the only Russian to serve prison time in the United States in relation to investigations of Russian political influence operations during and after the 2016 election. She now works for RT, a pro-Kremlin television channel.

According to social media posts by Ms. Butina and supporters of Mr. Navalny, the two had a face-to-face encounter that appeared to have been punctuated by mutual insults.

In 2018, Ms. Butina pleaded guilty in the United States to one charge of conspiring to act as an unregistered foreign agent, sometimes called “espionage light.” Prosecutors accused Ms. Butina of having befriended Republican Party politicians and leaders in the National Rifle Association while sending reports back to Russia. She served most of her 18-month sentence and then was deported.

Earlier in the week, Ms. Butina taunted Mr. Navalny in online posts for speaking out about his deteriorating health in the Russian prison, Penal Colony No. 2, known by its initials IK2, in the Vladimir Region east of Moscow, implying that American prisons are worse. “Are you a man?” she wrote on Wednesday.

Her visit on Thursday seemed intended to deliver the same message in person, according to the account posted by Mr. Navalny’s supporters.

“Instead of a doctor, today the miserable RT television propagandist Butina arrived, accompanied by video cameras,” said Mr. Navalny’s Telegram channel. “She was yelling that this is the best and most comfortable prison.”

Mr. Navalny narrowly survived a poisoning with a military nerve agent last year and was medically evacuated to Berlin. He returned voluntarily to Russia in January and was arrested at the airport. In February, he was sentenced to more than two years in prison.

In prison, Mr. Navalny has suffered from back and leg pain that has not been diagnosed, according to his lawyer, who said Mr. Navalny could not rule out lingering effects of the poisoning. Mr. Navalny has said that he has a herniated disk from riding in prison transport vehicles and that he is losing feeling in both legs.

He has also suffered from sleep deprivation as the guards wake him hourly at night because he has been categorized as a flight risk, despite returning to Russia voluntarily to be arrested, he and his lawyers have said.

Mr. Navalny’s supporters say the prison authorities are intentionally subjecting him to a drawn-out torment while also quickly tallying enough petty infractions, such as rising 10 minutes late or wearing a T-shirt to a meeting with his lawyers, to send him to a punishment block if they choose. It is a painful example for other Russian dissidents to watch.

On Tuesday, President Emmanuel Macron of France and Chancellor Angela Merkel of Germany raised concerns about Mr. Navalny’s deteriorating health during a three-way video conference with President Vladimir V. Putin of Russia. The Kremlin has said Mr. Navalny receives adequate health care.

On Wednesday, Mr. Navalny declared a hunger strike until he is allowed a visit with a specialist doctor.

The response prompted Ms. Butina’s taunting commentary online that was then delivered in person at the prison.

“A new approach for Navalny, a hunger strike,” Ms. Butina wrote on Telegram, the messaging service, on Wednesday. “It’s as old as the world.”

She wrote that his intent was to draw attention abroad and that others in Russian prisons had tried this before. “Look what little poor ones we are,” she wrote of what hunger strikers intended to convey.

“Lyosha, are you a man or not?” she wrote, referring to Mr. Navalny by a diminutive of his first name. “I’m tired of the complaining. He is in one of the best penal colonies in Russia.”

Ms. Butina in posts on Friday said that Mr. Navalny, in her view, looked hale and hearty. She said the warden had told her Mr. Navalny was refusing medical care from prison doctors.

“Navalny walks absolutely normally,” she wrote after the visit. “He doesn’t look like a person ‘not allowed to sleep,’ and I can judge from my time in prison in the U.S.A.”

In describing their encounter, Ms. Butina wrote that Mr. Navalny had been standing in a line of prisoners. When he saw her, she wrote, he “immediately hurled insults.” She also wrote that she asked him: “Do you know the difference between a prison and a resort?”

Ms. Butina served a portion of her sentence at the Federal Correctional Institution in Tallahassee, Fla. In a memoir published after her return to Russia, Ms. Butina wrote that she had been horrified to meet transgender people in the penitentiary and had once been sent to solitary confinement.

Mr. Navalny’s version of the encounter with Ms. Butina, according to his Telegram channel’s posting, differed on what was said, but at least seemed consistent with her assertion that it was an insult-fueled exchange.

Mr. Navalny “for 15 minutes lectured her before a line of convicts, calling her a parasite and servant of the government of thieves,” according to the posting.

Precisely how this information was obtained and posted to his Telegram channel is not clear. Mr. Navalny has conveyed messages through lawyers in the past that others post under his name.

RT, the television channel formerly known as Russia Today that Mr. Navalny’s aides said had dispatched Ms. Butina to his prison, did not respond to a query about the visit.