A Moscow district court issued the sentence on charges of recurrent organization of an unauthorized rally, Russia's Tass news agency reported.
The protests initiated by opposition leader Navalny, who was detained nearly immediately after the appearance at the rally, but later released until the hearing.
Rallies across Russian Federation were organised for May 5, ahead of Putin's inauguration for a fourth term as president.
"I consider the detention was illegal, my rights were violated", Navalny said in court, arguing he had a constitutional right to hold a protest.More news: Gwyneth Paltrow showed a rare photo of decheri
The Moscow City Court told Interfax that the terms of arrest Navalny was sentenced to do not add up.
The anti-corruption campaigner, who has become Putin's most visible foe, has served several weeks-long jail terms for organizing protests.
Almost 1,600 protesters were detained in 27 cities across Russian Federation.
Around one in 10 of those arrested were minors, according to the organisation.More news: NC Representative calls teachers "thugs" ahead of May 16 rally
Putin, 65, won re-election overwhelmingly in March, extending his grip over Russian Federation for six more years - a tenure of 24 years that would make him Moscow's longest-serving leader since Soviet dictator Josef Stalin.
The court also was expected to rule on another administrative case against Navalny involving charges of disobedience to police, which carries a punishment of 15 days in prison.
"They gave me 30 days, and the "trial" is still going", Navalny tweeted after the initial ruling.More news: The Honor 10 is practically just a cheaper Huawei P20