Stanislav Zimovets, 32, was convicted of having thrown a fragment of brick at a policeman during an anti-government demonstration in March.
"The fact that he acknowledged his guilt is perhaps what motivated the severity" of the sentence, his lawyer, Svetlana Sidorkina, told AFP. It might also have been meant as a warning to others, she added.
Yesterday, the Moscow courts jailed two other demonstrators convicted of assaulting police for a year and a half and eight months respectively, said Sidorkina.
A fresh call for protests by Navalny brought thousands of people out on to the streets in several Russian cities on June 12. But most of the 1,720 people detained across the country received only light fines.
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Earlier this month, Navalny himself walked free from jail after serving 25 days for having organised unauthorised protests against President Vladimir Putin's rule.
Navalny wants to run for president in 2018, but the electoral authorities said in June that his five-year suspended sentence for embezzlement meant he was ineligible.
Both the United States and the European Union have condemned the arrests of the protestors as a violation of basic human rights.