“Another shameless and lawless decision by Putin will not silence Ilya and should not frighten the honest people of Russia,” Navalny said in comments posted on Instagram after the verdict.
Navalny, 46, is serving a nine-year sentence for embezzlement, which is widely seen as politically motivated. Their political organizations have been declared illegal.
Others also praised the opposition politician for his stand.
Science fiction writer Dmitry Glukhovsky tweeted, “There are laws that should not be respected.”
“All over the world, Yashin is a hero and a prisoner of conscience, and his judges are criminals.”
Yashin refused to leave Russia after Putin sent troops into Ukraine on 24 February and regularly denounced the Kremlin’s invasion of Ukraine to his 1.3 million subscribers on YouTube.