Kremlin critic Alexei Navalny confirmed his arrival at an Arctic prison and said he was in good spirits.
Contact with his team has been lost since December 6, after he was transferred from another prison to eastern Moscow.
But on December 25, Navalny's spokeswoman, Kira Yarmysh, said he had been found.
He is considered the most vocal opponent of Vladimir Putin, and has been imprisoned since 2021.
Navalny confirmed on Tuesday that he had been transferred to the IK-3 penal colony nicknamed the “Polar Wolf” in the northern town of Kharp, about 1,900 kilometers northeast of Moscow.
IK-3 prison, located in the Yamalo-Nenets Autonomous Oblast, is seen as one of Russia's toughest prisons, and most of those detained there have been convicted of serious crimes.
Navalny wrote on the website I think it will be found before “mid-January”.
Navalny was previously detained in Melikhovo, 235 kilometers east of Moscow. He faces a 19-year prison sentence on charges of establishing and financing an extremist organization, charges he has always denied.
He noted that it was moved first to the capital, then east to the Ural Mountains region and then again to the west, before being moved north of the Arctic Circle.
The United States said that while it welcomed reports that Navalny had been located, it remained “deeply concerned” about his safety and conditions of detention.
Yarmysh said Russian authorities were determined to isolate Navalny and “try to make his life as unbearable as possible.”
She added: “This colony is very far away, it is very difficult to reach it, and for lawyers, it will be very difficult to go there and see Alexei.”
Ivan Zadov, Navalny's aide, said the prison change showed how “the regime deals with political prisoners and tries to isolate and repress them.”
His team's anxiety increased after he failed to appear in several court sessions.
Mr. Navalny made his name as an anti-corruption activist, amassing millions of views for his video investigations.
He appeared to be a charismatic activist, the only Russian opposition leader able to mobilize people in large numbers across Russia to participate in anti-government protests.
But in 2020, he was poisoned in Siberia with what Western laboratories later confirmed was a nerve agent.
He was treated abroad. Upon his return to Russia in January 2021, he was immediately arrested.
Navalny maintains his social media presence with the help of his staff and lawyers.
“Travel specialist. Typical social media scholar. Friend of animals everywhere. Freelance zombie ninja. Twitter buff.”