Patriarch Kirill told a congregation at Kazan Cathedral in Moscow's Red Square on July 21 that such recognition "means people are choosing a path of self-destruction."
The patriarch criticized the states that have legalized same-sex marriage, saying that "some countries approve their choice in favor of sin and justify it by law."
Kirill also said he supports a ban on "homosexual propoganda" recently signed into law by President Vladimir Putin.
Homosexuality was decriminalized in Russia in 1993 but remains stigmatized in a country where the Orthodox Church still holds considerable influence.
Gay-rights parades are forbidden and activists have been brutally attacked.