Russia's media watchdog, Roskomnadzor, has blocked access to two of the world's largest pornography websites.
Internet service providers had until Tuesday to implement the ban.
The sites now redirect to a message explaining they have been blocked "by decision of public authorities".
In 2015, the authorities banned 11 popular pornography websites, saying many failed to protect children "from information harmful to their health".
Analysis by Vitaliy Shevchenko, BBC Monitoring
The decision was made following two separate court rulings, which said the websites "spread pornography".
Sexually explicit content is not outlawed in Russia, but the law bans "the illegal production, dissemination and advertisement of pornographic materials and objects".
It is not uncommon for Russia to ban websites. The government agency Roskomnadzor maintains a blacklist that now includes thousands of them.
Access is usually blocked for violating the notoriously vague extremism legislation or child protection laws.
Websites critical of the Kremlin have been blocked too, and at one point Wikipedia was blacklisted.
Following the 2015 ban, one woman asked Roskomnadzor on Twitter whether it could recommend an alternative.
The agency replied: "You can meet someone in real life."
On Tuesday, it said its earlier tweet was "still relevant".
However, open rights campaigners have warned that local bans can be defeated.
"Blocking porn is the fastest way to ensure widespread adoption of censorship circumvention in your country," said Eva Galperin, global policy analyst at the Electronic Frontier Foundation.
One of the affected porn sites offered Roskomnadzor a premium subscription to its service, in exchange for lifting the ban.
The government agency said it was "not in the market" for such an offer.
Nadya Tolokonnikova, lead singer from punk band Pussy Riot, joked that the ban was "a blow below the belt".