The Russian opposition leader Aleksei A. Navalny has been moved to a prison north of the Arctic Circle but “is doing well,” his spokeswoman said on Monday, ending a 20-day mystery over his whereabouts that had many supporters fearing the worst.

“We have found Aleksei,” the spokeswoman, Kira Yarmysh, said on social media. “His lawyer saw him today.”

After an exhaustive, frantic search of Russia’s prison system after his Dec. 5 disappearance, Mr. Navalny’s exiled aides said on Wednesday that they had located him in a penal colony in the Yamalo-Nenets region of the Russian Arctic. Ivan Zhdanov, the head of Mr. Navalny’s anticorruption foundation, described the facility as one of Russia’s “most northern and most remote” prisons.

The penal colony, known as IK-3, in the Arctic settlement of Kharp, is about 1,000 miles northeast of the prison where Mr. Navalny had been held for much of the time since his arrest in early 2021.

“Aleksei’s situation is a clear example of how the system treats political prisoners, trying to isolate and suppress them,” Mr. Zhdanov said on social media.