Safronov, a former reporter for the Kommersant and Vedomosti newspapers who later worked as an adviser to the head of Russia's space agency, was arrested in 2020 and accused of disclosing classified information.
He was sentenced in September last year, in what his supporters called a draconian ruling that showed the absence of media freedom in Russia. He was accused of handing military secrets to the Czech Republic, a NATO member, but his defence team said the case was revenge for his journalistic reporting on Russian plans to sell fighter jets to Egypt.
Safronov was transferred in February to a high-security prison in the Krasnoyarsk region of Siberia, and was not present for Wednesday's Supreme Court decision in Moscow. On Monday, Russian opposition politician Vladimir Kara-Murza lost an appeal against a 25-year sentence for treason and spreading"false information" about Russia's armed forces by speaking out against Moscow's actions in Ukraine.