In a blog post, the Kremlin critic said he phoned a man named Konstantin Kudryavtsev, who he said was a chemical weapons expert with the FSB domestic intelligence agency.
Russian opposition leader Alexey Navalny has said he tricked a security agent into admitting the Federal Security Service (FSB) sought to kill him this summer and laced his underwear with novichok poison.
Navalny, one of President Vladimir Putin's most outspoken critics, was airlifted to Germany for treatment in August after collapsing on a plane in Russia. Germany has said he was poisoned with a Soviet-style Novichok nerve agent in an attempt to murder him, an assertion many Western nations accept.
Navalny declared his poisoning case solved earlier this month when a joint media inquiry said it had identified a team of assassins from Russia's FSB security service.
'He confessed everything'
In a video published on his website, Navalny said he had phoned up some of his alleged poisoners before the joint investigation was released and posed as an aide to the secretary of Russia's Security Council.
The Kremlin critic said he phoned a man named Konstantin Kudryavtsev, who he said was a chemical weapons expert with the FSB domestic intelligence agency.
"I called my killer. He confessed everything," Navalny said on Twitter, a claim rejected later Monday by the FSB.
Kudryavtsev spoke to him for 49 minutes and appears to have been part of the clean-up team, Navalny said. Navalny published a recording and transcript of the conversation.
Reuters could not independently verify the fact of the phone call or the identity of Navalny's interlocutor.
The Kremlin has repeatedly rejected any suggestion that Russia tried to kill Navalny.
"The video clip with the phone call is fake," the FSB said, according to the state TASS news agency, calling Navalny's investigation into his alleged poisoning a "planned provocation".
Foiled underwear plot
In the video, Navalny can be heard asking a voice on the phone. "Why did nothing work out?"
"Well, I've asked myself this question more than once," the voice responds.
In the audio recording, the voice on the other end of the line initially sounds hesitant and cautious but eventually tells the story.
When asked why Navalny survived, the voice says it was probably because the plane he was on at the time made an unplanned emergency landing and he was treated professionally and quickly by Russian medics.
"If it had been a bit longer, then it's possible it would all ... have ended differently," the voice says.
In the recorded comments, the agent said an attacker had placed the poison along the inner seams of a pair of Navalny's underwear.
He detailed how he and another FSB agent had flown to Omsk after the poisoning and removed any trace of the poison.
On Monday, Navalny's associate Lyubov Sobol went to a building on the outskirts of Moscow where the Bellingcat report said Kudryavtsev lives.
According to a video she live-streamed on her Twitter account, police surrounded her car and later detained her.
Journalists also arrived at the scene and rang the doorbell to Kudryavtsev's reported apartment, but no one came to the door.
Putin last week dismissed media reports about Navalny's poisoning, saying they were made up of information provided by US intelligence services and were an attempt to smear Putin himself and make Navalny seem more important than he really was.
Navalny is convalescing in Germany, but has said he plans to return to Russia at an undisclosed date.