The supreme court of the Karelia northwestern region "annulled the verdict of acquittal" of Yury Dmitriyev, whose trial ended in April, lawyer Viktor Anufriyev said.
"Now the case will be sent up for a fresh trial," he said.
The lawyer condemned the decision as "unlawful and not based on criminal legislation."
Dmitriyev, a historian who has spent decades locating and exhuming Stalin-era mass graves, was released after his surprise acquittal, which his supporters hailed as a rare victory for the country's embattled rights activists.
The case centred on naked pictures that Dmitriyev had taken of his adopted daughter, in order he said to monitor her development as she had been malnourished. Experts said they were not pornographic in nature.
Dmitriyev has reacted "philosophically" to the abrupt reversal in his legal case, his lawyer said Thursday.
"He takes this decision philosophically, he has experienced life and has seen even worse," Anufriyev said.
Following his closed-door hearing in Karelia's main city of Petrozavodsk, Dmitriyev shouted in triumph: "I am free," after spending more than a year in pre-trial detention.
At the time, the 62-year-old, who is the head of rights group Memorial's branch in Karelia, praised the ruling as "just".
He was however found guilty of firearm possession and released on conditions that limited his freedom for the next three months.
The verdict on arms possession has also been annulled, Dmitriyev's lawyer said Thursday.
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