City hall announced on March 11 that St. Petersburg Governor Georgy Poltavchenko had signed the law on March 7.
The regional law -- which applies only to St. Petersburg -- threatens fines for anyone found "making public actions among minors for the propaganda of homosexuality, lesbianism, bisexuality or transgenderism."
Another article levies fines for "propaganda of pedophilia."
The law comes into force 10 days after being officially published.
Human rights groups had called on Poltavchenko to veto the law on grounds that it appears to equate homosexuality with pedophilia and risks being used arbitrarily by authorities to crack down on gays.