Kremlin critic Alexei Navalny has told his supporters not to worry about him on social media.
Reappearing for the first time since December 6, he confirmed his arrival at the Arctic Prison and said he was in good spirits.
His team did not communicate with him when he was transferred from another prison east of Moscow.
Considered the most vocal opponent of Vladimir Putin, Mr Navalny has been jailed since 2021.
Writing on X, formerly known as Twitter, that he is the “new Santa”, Mr Navalny confirmed on Tuesday that he had been transferred to the IK-3 penal colony, nicknamed the “Polar Wolf”, in the northern city of Corpus Christi. 1,900km (1,200 miles) northeast of Moscow.
IK-3, in the Yamalo-Nenets Autonomous District, is considered one of Russia's toughest prisons, and most of the prisoners held there are convicted of serious crimes.
Mr Navalny said he was taken “with such caution” and “on such a strange path”, adding that he was surprised when told his lawyer was there to see him because he did not think he would be found “until the middle”. January”.
But Mr Navalny's chief of staff, Leonid Volkov, told the BBC the opposition leader's concerns were greater:
“We are worried. We will not forget for a second that he is in the custody of the people who tried to kill him three and a half years ago and that he is Putin's personal political prisoner,” Mr Volkov said. .
“This forced visit is another proof,” he added.
Mr Navalny was previously held in Melekovo, 235 km (145 miles) east of Moscow. He faces 19 years in prison for founding and financing a terrorist organization – charges he has always denied were politically motivated.
He can only post on social media when he has access to his lawyers and last month three members of his legal team were blacklisted.
Mr Navalny suggested he was moved first to the capital, then east to the Urals Mountains and west again, before being moved north of the Arctic Circle.
“My 20 days of transportation have been very tiring, but I am still in good spirits to suit Santa Claus,” he wrote.
The Kremlin critic continued tongue-in-cheek: “Because I'm Santa Claus, you wonder about the presents. But I'm a special regime Santa Claus, so only those who have behaved very badly get presents.”
While welcoming the news that Mr Navalny had been found, the US said it was “deeply concerned” about his well-being and detention conditions.
Kira Yarmish, Mr Navalny's spokeswoman, said Russian authorities were isolating him and “trying to make his life as unbearable as possible”.
“This colony is very remote, very difficult to access and for lawyers, it will be very difficult to go there and see Alexey,” he added.
Ivan Zadov, an aide to Mr Navalny, said the prison reform demonstrated “how political prisoners are treated, trying to isolate and suppress them”.
His team became increasingly concerned as he failed to appear at several court hearings.
Mr Navalny earned a reputation as an anti-corruption campaigner, garnering millions of views for his video interrogations.
A charismatic campaigner, he appeared to be the only Russian opposition leader capable of mobilizing large numbers of people across Russia to participate in anti-government demonstrations.
But in 2020, he was poisoned in Siberia by what Western labs later confirmed was a nerve agent.
He was treated abroad. He returned to Russia in January 2021 and was promptly arrested.