Nikolai Alexeyev has been fined 5,000 rubles for "propaganda of homosexuality" to minors.
The 34-year-old lawyer was detained for holding up a banner outside the city hall that read "Homosexuality is not perverted. What's perverted is hockey on grass and ballet on ice," a quote from a Soviet film star.
Alexeyev, who has organised numerous unsanctioned gay pride marches in Moscow, says he will appeal and contest the law at the Constitutional Court and the European Court of Human Rights.
"I think it will be viewed very negatively and it will show the absurdity of what's going on in 21st-century Saint Petersburg," he said, calling the law "absolutely arbitrary."
He said that he was charged with propaganda to minors even though there were no children around him, with police gathering witness statements from people sitting in a nearby park who had not even seen his poster.
Other activists have been charged under the law, but Alexeyev is the first to be sentenced.
Similar laws have been introduced in three other regions in Russia, while a draft proposal has also been put forward to make the rules national.