SOCHI, Russia—Russia banned adoptions by single people from countries where same-sex marriage is legal, regardless of the prospective parent's sexual orientation. The new measure comes into law in the middle of a Winter Olympics already steeped in controversy over the country's attitude toward gays.

The decree, signed by Prime Minister Dmitry Medvedev and posted on the government's website Thursday, also reiterates a ban on adoptions by same-sex couples who are in unions "recognized as marriages and registered in accordance...