A court in Moscow has approved a request from the Russian media regulator to block the Telegram messaging app immediately.
The media regulator sought to block the app because the firm had refused to hand over encryption keys used to scramble messages.
Security officials say they need to monitor potential terrorists.
But the company said the way the service was built meant it had no access to customers’ encryption keys.
Telegram had missed a deadline of 4 April to hand over the keys.
Russia’s main security agency, the FSB, has said Telegram is the messenger of choice for “international terrorist organisations in Russia”.
A suicide bomber who killed 15 people on a subway train in St Petersburg last April used the app to communicate with accomplices, the FSB said last year.
The app is also widely used by the Russian authorities.