Kremlin critic Alexei Navalny confirmed his arrival at the Arctic prison and said he was in good spirits.
His team has not been in contact with him since December 6, when he was transferred from another prison east of Moscow.
But on December 25 Navalny's spokeswoman Kira Yarmish said he had been found.
Considered the most vocal opponent of Vladimir Putin, he has been imprisoned since 2021.
Navalny on Tuesday confirmed his transfer to the IK-3 penal colony, nicknamed the “Polar Wolf,” in the northern city of Karp, about 1,900 km (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.
Writing on X, formerly known as Twitter, Mr Navalny said he was taken “with such caution” and “on such a strange path” that he was surprised when he was told his lawyer was there to see him. I think he will be found before “mid-January”.
Navalny was previously held in Melekovo, 235 km (145 mi) 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 three members of his legal team were blacklisted last month.
Navalny suggested that he moved first to the capital, then east to the Urals Mountains, and west again, before being moved north of the Arctic Circle.
While welcoming the news that Mr Navalny had been found, the US said it was “deeply concerned” about his well-being and detention conditions.
Ms Yarmish said the Russian authorities planned to isolate Mr Navalny and “try 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, Navalny's aide, 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. Returning to Russia in January 2021, he was immediately arrested.
Mr Navalny maintains a social media presence with the help of his staff and lawyers.