The data included email addresses and passwords that had been stored without any protection, a security firm said.
Leaked Source said the massive cache of credentials dated from 2012 but had only now been leaked and put online.
And it had come from a hacker who had supplied security firms with 43 million user names from music service Last.fm.
Rambler has been described as the Russian equivalent of Yahoo as it offers email services as well as acting as a news and content hub for its users.
"We know about that database," said the service in a statement.