Alexei Navalny was poisoned via a water bottle in his Siberian hotel room, according to the Kremlin critic’s entourage.
The claim was made via an Instagram post on the politician’s account on Thursday morning.
The politician’s team say they found traces of the novichok nerve agent in a bottle in his room in Tomsk.
Mr Navalny is being treated in a German hospital after he was taken ill on a flight on 20 August.
It was previously said that he could have been poisoned through a drink he consumed at the airport before his journey.
According to the German authorities, the Putin critic was targeted with novichok – the same nerve agent used against Sergei Skripal in Salisbury in 2018.
Laboratories in Germany, France and Sweden have confirmed that he was poisoned with the nerve agent.
Russia has said it sees no evidence of poisoning in the Navalny case.