He was fined 500,000 roubles (Â£6,000, $8,000) but avoided prison.
The case centred on a blog post he wrote on LiveJournal in 2015 with the headline Wipe Syria off the Face of the Earth.
Nosik was charged with inciting racial hatred after writing that he supported those who were bombing the country.
Judge Yevgeni Naydenov said: "Nosik's actions sought to incite enmity and hatred towards Syrians, a group of people based on ethnic and geographic characteristics."
Nosik wrote: "My opinion on this issue is that my statements do not contain any extremist views... I positively do not see any extremism in the fact that I will not feel sorry for the Syrian state."
The case has divided opinion in Russia, with some accusing the state of double standards while others have criticised Nosik for his controversial views.
Nosik, a former journalist, is considered by many in Russia to be an internet pioneer and was behind the development of several popular websites.
The country's extremism charge (Article 282 of the criminal code) is widely applied.
In 2015, a 23-year-old Muslim woman from Pervouralsk was convicted of extremism after saying New Year's Eve should not be marked because Santa Claus was "pagan".
The trial of Anton Nosik is a high-profile example of a recent trend in Russia that has seen increasing numbers of bloggers and social media users put in the dock for posting or simply reposting material online.
A few years ago, most cases of this sort were brought against people actually inciting violence.
These days, the defendants are often simply critics of Kremlin policy, especially in regard to Moscow's intervention in Ukraine.
Unlike Nosik, several of them have received substantial prison sentences, of up to five years.
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Anton Nosik, a well-known Russian blogger and internet entrepreneur, has been found guilty of extremism by a Russian court.